Adding some extra options for flying movement, to allow faster movement when descending.
A falling creature moves vertically downwards at a speed of 600ft in the first round, and 2,000ft in subsequent rounds.
Once you begin falling, it's a Fly DC 10 check to recover, but you are considered to have been plummeting at high speed the previous round (see below).
When flying, for every 5ft you descend you can move an extra 5ft horizontally or downwards for free.
For every 5ft you ascend, you must also move 5ft horizontally. This costs 10ft of movement.
If you want to avoid the 5ft of horizontal movement when ascending, you must make a Fly check DC 20, but it still costs you 10ft of movement. You only make one check per round, at the point you first want to avoid the horizontal movement. If you fail by 5 or more, you fall 600ft if it's in your first move action, or 300ft if it's part of your second move action.
|Flying maneuver||Fly DC|
|Move less than half speed and remain flying||10|
|Turn greater than 45° by spending 5 feet of movement||15|
|Turn 180° by spending 10 feet of movement||20|
|Fly up at a greater than 45° angle||20|