I run home brew 14% synthetic oil, 2% castor, 14% nitro. In the past I have run Saito engines on zero to 30% nitromethane. Finally settled on the 14% as a good all around fuel for engines from the 30 single to the 170 R3 radial and the 182TD twin.. They all idle well, transition well and make excellent power. Needle settings are easy too.
The nitro affects the entire range of engine operation, not top speed alone. It can improve idle as well as transition. The 10% to 15% range usually provides the best results for the money at the idle and transition while top end improvements show up with the 20% to 30% cans. BTW, Make no mistake, a Saito can really sing with that 305 dose of nitromethane
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You will do just fine with 15%.