There is a headspeed calculator that will give you your headpeed.
You need to know your motor's KV rating, and the number of cells you are using, then you can adjust the pinion gear size to get the proper headspeed at 100% throttle.
www.e-helis.com has one I think, you just click on the motor, batt, pinion, and type of heli and it gives you your headspeeds.