Does a Yeld spyrer's fly ability work just vertically, or on a hypotenuse?
You measure from the base of your starting position to the base of where your movement ends. The movement must finish at an appropriate point (it cant be mid air for example). If that end point is higher than your starting point then its 2", if its lower its 1/2" 

So for example, if you have movement 8, you have three options


Complete a move up to 4" ending above your starting position.
Complete a move up to 8" horizontally (normal move)
Complete a move up to 16" ending your movement below your starting position.