1/23/2020: I revised these again. Short version, movement rates are fixed at 10 and 40 base move.
Here is another spell I've merged in my game.
Right now, Flight and Hawk Flight are separate spells. I've merged them into one spell.
These two spells don't really do anything different, and they had some odd holdover mechanics from 1e-3e.
Instead of calculating Encumbrance with this slightly odd effect (normal for Flight, x4 for Hawk Flight), you can just calculate it normally. The wording doesn't really make sense for 4e, with a multiplier instead of a flat subtraction for encumbrance.
So a person with Light Encumbrance (x0.8) flies at 8 with Flight, and 32 with Hawk Flight. With Heavy (0.2!) you're at 2 and 8, respectively.
They do have different Time to Cast, but really, is it worth charging an extra second to cast the spell? I don't think so.
So here is what I did to it:
Gives the subject the Flight advantage (p. B56), with an air move equal to 2xBasic Speed. For an extra cost (see below) you can fly at an air move equal to 8xBasic Speed. Encumbrance reduces flight speed normally. Flying characters may have a combat advantage (see p. B402).
Cost: 5 to cast, 3 to maintain. 8 to cast, 4 to maintain for 4x speed!
Time to cast: 2 seconds.
Prerequisites: Magery 2 and Levitation.
Hawk Flight is also extremely maneuverable - it's not Enhanced Move (Air) but just a flat speed increase. This is pretty damn powerful. Another way to modify the spell is to say you can add levels of Enhanced Move (Air), but you'd want to reduce cost as this reduces its utility and effective speed in combat. Probably change the 8/4 to 6/3 or 7/3 in that case.
It would probably be fair to just say 4x move is x2 cost, but 10/6 is brutally expensive for what you get out of it. So I'm leaving that alone.