I'd stick in the 76 to 78 range.
In the old days
with brushed motors and nimh you needed small pinion, personally now I never ever run anything faster than a 6.5 so never on much smaller than a 22 pinion which fits easily. Think I can get 19/78 on mine.
Since the last meeting I have a 76 on in fact it's currently got 23*76 on a 7.5 motor if I'm not mistaken.