the RB and novarossi 21 motors are set from the factory to run up to 25% nitro without adding shim. Add .1mm shim for every 5% increase in nitro. (I emailed RB and that was what they told me).
I shim all my motors, 21 and 12 to run 30%, that way all I need to get is 30%.
25% is kind of hard to get. 20% is good for offroad but I can't use it for on road. One time I grabbed the wrong %, the other time my 20% sat for one year and I had to dump it.