In-game, it's basically said that even a C rank Noble Phantasm is the equivalent of a powerful normal attack - the idea being that the standards for them are higher. This was something along the lines of a C Noble Phantasm being the equivalent of an A or an A+ attack. Legendary items and their true attacks are just going to be inherently special in terms of the kind of damage being dealt as well as whatever unique characteristics they have. Noble Phantasm ranks are going to be unique so they shouldn't be used as a standard for judging other powerful artifacts. Hell, unlike Mystic Codes and/or (ancient) Conceptual Weapons, Noble Phantasms themselves for the most part aren't even supposed to be a part of the modern world (with some exceptions). Most NPs are going to be existentially elevated weapons (the majority formerly being what's now called Mystic Codes) through the exploits of "heroes" with some of them being direct products of planet terminals (gods, nature, tree of mana, whatever)....probably.

Now, Invisible Air's air blast is pretty much the same as Forte's air wand effect. That along with what other abilities she has is supposed to be around Burier-class.

1/02 update
IA windblast appears to be faster than Forte's windblast (supersonic vs sonic), which would put it some ranks below.

This would put the air wand/sword at a level below the average Noble Phantasm, but still powerful in respects to the modern world. A+, A, still is high and pretty much generally the peak; ie, there isn't supposed to be anything beyond high thaumaturgy other than magic.

Add that to the fact that Ciel can't manage anything more than a defensive fight against a Servant with an average Noble Phantasm (B rank, mainly pure damage-related (numerical) + special effects).

Assuming that Ciel is physically equivalent (which she probably is; at the very least, she should be sufficiently higher than Bazett in straight combat in avoiding getting killed within one minute) to Servants, this would probably put the Seventh Scripture as the equivalent of a below-average Noble Phantasm, just like Forte's wind sword. Or, assuming if Ciel's overall skill is inferior, the Seventh Scripture should at the most be equivalent to an average Noble Phantasm.

Note that both weapons are supposed to be relatively high in respect to value.

If we're going to be generous and assume that normal A+ = Noble Phantasm C, then a chart for damage would come down to something like:

NP ranks
A: 167 (with a min of 83)
B: 133 (with a min of 67; note that setting-wise, rank B NP should definitely be one-step higher than the normal values in the A to A+ range - best to keep just one standard instead of any sort of variations)
C: 100 (with a min of 50)
D: 67 (with a min of 33)
E: 33 (with a min of 17)

A+: 332 (with a min of 167)
A++: 501 (with a min of 250 - damage radius being able to take out some thousands of corpses)
A+++: 668 (with a min of 333)

Typical Ea: 800 (should be seen as the general value when the full name is released, however, since Ea stats include MGI at random, average amount should be even higher); consciously lowering output is possible (although even at lowered strength, even using several NPs as disposable shields against it will still result in critical injuries)

True Ea: Over 4000. ..... At the very least, more than capable of taking out over a thousand targets

Whereas normal ones would be (only in respective to weapon damage, not in terms of physical stats and how that translates into damage in conjunction with weapon; unrelated but Archer was able to kick Shirou easily 5 meters away)
A: 50
B: 40
C: 30
D: 20
E: 10

Weapon-power (as in just plain-out special/normal attacking) level and their positions in the modern world could be represented as being the following (given that there aren't any real figures for them). Of course only (really, really stupid) speculation, but one that could help at getting a general idea of respective power of individual weapons.

Ultimate level

??? - Slash Emperor/Avalon
??? - Notes-era Longinus (power seems to have been boosted since the big definer of natural life-span is dead)/Ea (however, only true Ea - over 4000 level damage compared to the uncharged and boosted level - regular Ea would probably still be over even A+++)
??? - Ultimate-tier magic (probably overlaps with A++ area)

Phantasm level (below anti-world)

A++ = divine spirit level attack/Excalibur/Book of the Stomach World (supposedly able to definitely destroy any ONE (just one) opponent, but still a use and discard weapon and probably can't take out the modern Big Three - given that Nahat is doppelganger-related, actual usage might be a more powerful and bigger version of Delusion Heartbeat)
A+ = lower-tier magic/Bellerophon/lower-tier dragon attack/Tohsaka Zelretch/Caster's laser rain (in terms of pure output; each light bolt is the equivalent of an A rank spell; considering rapid succession and volley effect, damage should at least be equal to A+ within the space of 15-30 seconds)
A = exceptional NP, HP for one Berserker life (assuming it would take less than 4 A rank (or higher) jewels to blast off his head)
B = average NP; high-ranking conceptual/destiny interference types, destiny-interference NPs (Gae Bolg - however, pure damage ignoring causality effect and critical hit to heart is at B level; thrown Gae Bolg does have numerical damage surpassing A rank (B+))

Below average elemental/transcendent/ether-filled era level (using normal rankings - anything below A (Scripture)-level should be judged as being insufficient for doing anything other than buying time against elemental-class opponents; further the following ranks would have a rank of C in NP terms or below). Seven may or may not be equivalent to an average NP depending on how one interprets the Servant vs. Ciel comment; skill variations should be possible to boost base damage - not up to the point of NP true name releasing, but still enough for some damage enhancement for melee type weapons (Tsubame Gaeshi by that farmer; interpretation of "normal damage" as more or less being represented as non-Noble Phantasm ultimate attacks could also be valid); augmentation of certain energy blasts by spells should be possible. Normal non-released damage for NPs aren't factored in, because frankly, I'm starting to get confused \o/

A to A+ (aka, C rank NP or border-line NP level) = Scriptures (some may make it to the average NP line)/Apocrypha/elite Lord-class level Mystic Codes/higher tier ten-count spells/below-average Noble Phantasms/non-Answerer Fragarach; again, only one-time use), Tsubame Gaeshi; (all thaumaturgy in this area still should technically be classified as rank A, although damage might surpass base 50), Delusional Heart Beat
A = one fully charged gem (10 years full of maryoku from a magus having a capacity of more than 20 times that of the average magus), typical ten-count (high thaumaturgy) spell
C = Ciel-boosted Black Keys (capable of destroying one normal Dead Apostle (emphasis on normal - not ancestor) 6 times over; rating is not taking into account key of providence effect on vampires though; base attack value able to penetrate concrete), Tohsaka-charged Azoth (only one-time use), Kiritsugu's guns, average limited-type Mystic Code
D = average Black Key (not taking account extra damage to key of providence effect), super Gandr, average amplifier Mystic Code, normal gun shot
E = non-Tohsaka Gandr

or, in regards to the last segment; normal attacks by NPs (in which case the above ought to be for instant "special" skill-based damage); energy/mana blasts from the above should fit under the same rankings in the below, while melee ones from above ought to factor in damage based on ultimate skill rather than normal attack....ah, what the hell!

A+ = axe club (factoring in Berserker's personal strength and mad enhancement)
A = normal Excalibur (any of the non-Shirou Sabers)
B = Gae Bolg, typical normal attack from vault weapon (not rain of Hougu) by Gil, chain nails (normal Rider), single dirk volley from True Assassin (factoring boost from throwing skill?), normal arrow volley (approx. 13; on the other hand, they still look like they do more damage than a dirk volley; probably because they explode? wouldn't dare to put them above B though) from Archer
C = Normal attack by Monohoshizao (factoring in skill; sword itself is rather fragile given that it's not a NP)
C to D (C-) = Individual Kanshou and Bakuya (both of them together probably ought to result into the equivalent of a lower B rank - Overedge might possibly be able to reach rank A)
E: Normal physical attack by Caster (Caster slap?)