Hi,
Recently i have tried the calculator by 'Mr Mel' and it's pretty good. here is the link:
http://heli.dacsa.net/calcv3/This will give you various calculations where you can input target headspeed/ and the main gear teeth count which is also available as a preset and you can input different motor Kv's to find the best match.
Also a good manual calculation where you can try different Voltage/Motor KV/Pinions to find a target headspeed.
Battery Voltage x KV of motor x # of teeth on pinion x % of efficiency of motor / main gear teeth...
Example of 600 with a Hacker A50-14S:
Example: 37v * 580kv * 15T * 85% / 116T = 2358 rpm (Goven'd @ 2250)
Hope this helps,
Cheers!
Ross