Gearing for a E-Revo w/10xl and 16 cells??

[squeeforever]

i will be running my E-Revo with a maxx tranny, 10xl motor, 9920, 16 ib 4200 cells, and revo slipper assembly with a 40 tooth mod1 spur. what would be a good place to start for gearing? i was thinking of 16 or 18 but dont want to buy a 16 or 18 and it be under of over geared since the pinions arent to cheap. any help would be greatly appriciated.