I don't think with the definitions you have now you need to explicitly tell IASA to measure anything. Just say territorial airspace ends where airfoil-based powered flight becomes impossible and be done with it.
Personally, I would delete everything in 4. 3 doesn't need clarification 4a/b) still clarifies nothing - that's what sovereignty means. You've already given nations control of their airspace, so let them have control. Once you've said nations have sovereignty over their own airspace the more you 'clarify' things the only thing you can possibly be doing is giving more control to the WA and the less to the nations. 4c is new and clearly illegal for the direct contradiction of Civilian Aircraft Accord, which you do have to actually read. 4d is not needed because it's already covered in CAA.
Moreover, if you just give nations control of their airspace (with the existing exceptions defined in CAA) you can also get rid of the confusing mess of 5b (I also have no idea why you'd pair it with 5A since they have nothing to do with each other). Who cares where airspace, the air above land that is not airspace for some reason, and actual land is. Their all the nation's territory that they can regulate as they please (subject to existing WA law).
Here's how I would organize things with a few grammatical corrections thrown in as well
1) Defines "Territorial airspace" of a nation as the portion of the atmosphere above the recognized land and water territories of a member nation, with the upper limit determined by where airfoil-based powered flight becomes impossible.
2) Mandates that all member nations shall have sole sovereignty over their territorial airspace, with the exception of any World Assembly regulations regarding civilian aircraft protections.
3) Defines "International airspace" as any portions of the atmosphere not above a nation's sovereign territory, with the lower limit determined by International Aero-Space Administration (IASA) based on stationary landmarks in the vicinity, and the upper limit determined by where airfoil-based powered flight becomes impossible.
5) Mandates that all aircraft registered under International Transport Safety Committee (ITSC) regulations shall have the right to fly in international airspace.
6) Prohibits any member nation from exercising control over international airspace.