Here's a little offset calculator. It works very well

http://www.gsal.org/tools/offset_calc.htm



This works also:
If you can stand the plane upright and secure it, set the engine on the firewall, put the cowl on and line the shaft up with the cowl opening. Carefully remove the cowl and mark your standoff locations on the firewall.
This is sometimes easier said than done, but if you don't have the elevators and rudder on yet it is pretty easy.