This is ALMOST like asking "what gear should I run"...there is NO way to answer it accurately online. Every track is different, even every motor can be different. People online can give you a general area to start but there is NO way they can pinpoint the optimum rollout.

Best bet is to ALWAYS ask the "fast guys" at your track about what roll-out they're running. Then don't be satisfied with that--do a little experimenting up and down from there because unless you're running the same motor (same brushes, same springs) etc as they are chances are even their optimum set-up isn't yours. Even driving style comes into it. Someone who treats the throttle like a switch will have to run a lower roll-out than someone who tips into it. The temp gauge (or well-calibrated finger) will tell you if you're approaching danger and the clock will tell you if you're making improvement, slowing down or staying the same.

Example. We race on a club track so we roll the carpet out for each event and create a different track each time. The straight lenght, which is pretty much always the criteria for online guesstimates, is always 65' yet my gearing runs over a 2-3mm rollout range depending how tight we make the infield. The key is figuring out where in that range to go within the, typically, one practice pack and 3 heats before the main while your competitors are too. At a permanent track once everybody has the gearing set you don't typically see big lap time drops between heats, for us it's not uncommon for the faster drivers to improve as much as .5 sec/lap over the day as they get the gearing and other set-up items dialed in on the fly. Then it's back to it for the next race. It never changes a lot so I almost always start out where I left off last race and adjust from there. I end up running Stock in the low 40's mm and 19T in the mid 40's mm and that ranges over about 3mm. On our layouts. With my set-ups. With my driving style. Add caveat of your choice here.