At first, I was running 64mm all the way around because the serpent 710 has a slight overdrive in the design that proved useful with equal diameters. To get a 1:1 ratio with the stock pulley, I'd have to do a 1mm split.
Now that I put the overdrive pulley (went from 22 to 21 teeth) I'm going to have to true my front tires down 1mm for now. I want to see the difference so I'll first try equal, then incrimentally going to a 2mm split. Gotta adjust the upstops if I go to a 2mm split so I am hesitant to do it.
The main reason I wanted to try the overdrive pulley was to get more throttle stability and pull the car through the turn with throtle steer. Especially since I'm running a locked front axle. I've messed around with different settings but with the track we're going to on sunday, more speed will be carried through the turns because of the larger lanes and layout. It's designed for 1/8 onroad. In the end, I may just go back to the stock pulley if I don't feel comfortable with the 21T on that particular track and layout.
And yes, the "end play" as RRR stated is very important to clutch engagement so it can be critical. Need to shim it to get proper tolerances.