What I say here is for Novarossi based engines shimmed from factory. It is a general rule of thumb on .12 engines that for every 5% nitro increase you add .05 of head shim.

However if you are running 30% nitro a .1 shim will work perfectly. 20% no shim needed.

There is a head clearance that you want to maintain however this involves actually knowing how to PROPERLY measure this.

John