Roll-out = Tire Diameter * PI / (Spur / Pinion)

Most guys running 1/12 will measure Tire Diameter in mm, which will give your roll-out in mm.

If you're going to run a 10.5, I've usually run it around a 60mm roll-out with a boosted ESC.

I made an excel spreadsheet I use to figure out gearing, PM me an email address and I'll share if you're interested.