Help gearing a tamiya f103gt

[lee1092]

i am getting a brushless system which is classed as a 13t and im going to get an 80(ish) tooth 64dp spur gear but i dont have a clue about what pinions to use, i know that ill probably have to do some fine tuning, but ive been told a way of roughly finding a close pinion quickly but i've forgot how to.
any help on finding a way of finding the pinion to use would be appreciated

thanks, lee