The Three F's: Flying, Fighting, and Falling*
Personally, I would ignore all real altitude restrictions, and simply classify them into "sections". For example, it might go:

Near Earth (they have to fly around trees)
Low Sky (meaning they avoid most trees)
Middle Sky (meaning they have to fly around high and low mountains)
High Sky (meaning they only have to fly around high mountains)

Climbing one category takes a cumulative number of rounds... one round from Near Earth to Low Sky, three rounds from Low Sky to Middle Sky, and six rounds from Middle Sky to High Sky. Losing altitude requires one round per category lost. A dive can be accomplished in one round per three categories (meaning a dive from High Sky takes 2 rounds, otherwise it only takes one), with damage multipliers based on the number of categories dropped through (so a dive from Near Earth doesn't do anything, one from Low Sky does double, Middle Sky triple, and High Sky quadruple damage).

Impose am altitude maximum on all the various flying creatures... bugs should be limited to Near Earth, Aarokcra to Middle (or maybe High) Sky, and Rocs definitely fly in High Sky.

Quick and dirty, easy to remember, and avoids all the pesky numbers that physics majors will whine about being wrong.

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*Hey, this is a family site. What did you think I was gonna say?