i run wildcat 65% nitro/18% oil in my cmb .21's. both greenhead LS & goldhead TT, they seem very happy with it, but so do my k&b .21 ob's. the smaller the engine, the higher the nitro they seem to like, imho.
I run 60% in my stock Cmb .21 w/ Seaducer hull and I have ran the motor for one and a half seasons without a problem. The boat even took the district 3 championship with that combo.
I generally run 60% in everything up to my .45's. The difference in performance of a .21 engine running 50% and 60% is astronomical. If you have only run your engines on 50% or less you have no idea what you're missing out on in terms of performance. The jump from 60% to 70% is like wise, but you sacrifice ease of operation because the needle settings are much more sensitive...But those little engines sure do sing pretty at full song on 70.