[Dandare]

A 24t pinion should be ok, I've also used a 27t. It does depend where (what type of track?) you're racing on. You can overheat a motor by maxing the rpm on long straights if under geared, likewise you can do the same by over gearing it on tighter track, it's a tricky one to figure out sometimes.

Regarding the slipper, I first set the diff tight enough so when you hold/lock one back wheel plus the spur gear you can almost not turn the other wheel, this is done with the slipper fully tightened. Then, for a base set up switch the car on, put it on the bench/ground, hold on to both back wheels & apply a constant steady throttle, you want the slipper set loose enough so the front wheels remain off the ground by 2-3"

As a note, you always want the slipper to slip before the diff does, or else you'll soon knacker the diff plates etc.

If you're using it on loose ground eg. dirt then you may want to loosen it further so you don't keep spinning out, on a higher grip surface eg. astro you'll tighten it slightly & maybe even add some weight on top of the servo.