the fd you found online is a suggested number. by going 88/20 to get 8.0 you are moving toward more top end speed. what you wanna do is go higher than 9.43 maybe even to the 10.0 range..it won't hurt the motor. so increase your spur gear or decrease your pinion or both. whatever is easier.

also you might want to tone down the motor by plugging in the esc to your computer and lower all the settings.