For 12th scale tires I just set the truer to the final diameter I would like and cut them down. For WGT I will do an intermediate pass that is about halfway between the initial diameter and my desired final diameter, but I don't think it is necessary for 12th scale.
I also run 42 mm as my absolute maximum size, so that is what, at least 4-5 mm cut down in one 'pass' (across the tire twice) depending on the initial tire size?
Chris Hillier | 5280 Raceway | www.5280raceway.com | Racing. Elevated.
Destiny | LRP America | Protoform | Xenon/JFT | Gravity RC | Motiv | Sanwa | Max.FX