I run my YS 120 on 15% without too much trouble, I did find the mid range a bit rough. I did improve things with tuning the regulator. Apparently the Japanesse only use 15% nitro fuel. I have also been told that CPLR uses 15%, but no idea if that is true or not.

With my 120 I will use 15% until I have used up the 3.5 gallons of prosynth I have got, then as I plan to get some more YS engines, I'll probably switch to 20%.