You won't be able to make it perfect by using a wait. I mean you can calculate how long it will take to rotate with time=(360/sprite.count)/speed, but since things update at 60fps you'll be off by 1/60sec or so which will add up. Plus, wait makes it harder to track what your events will do, which can make it hard to find the reason things aren't working.
Conceptually you just need to rotate it a set number of degrees and then stop. Maybe with an ease?
At any rate here is one way. The arrow keys change the goal angle, then the actual angle changes toward that. It will robustly stop at the correct angle.
dropbox.com/s/d9ljfr2sade5orp/revolver_menu.capx
There may be a simpler way too, but this should be simple enough to understand so you can use it in your project and customize.