I've been using 10% nitro. I tried 15% for a little while in my .46 but I noticed the decreased 'fuel mileage' over the power increase.

I think 15% is a good idea for competitive flying, but for sport flying there's no need. Nitromethane requires less oxygen than methanol to burn. As you increase the nitro content, you have to open your needles to allow more fuel into the engine. You get an increase in power due to the larger amount of fuel burning, not because nitromethane has more power. Your flight times will be shorter with higher nitro content. 10% is also less expensive to begin with. If 5% was available around here, I MIGHT try it, though 10% is probably a happy balance between cost, fuel savings, and performance.

As was stated above, you actually don't need ANY nitro to run an engine. However, the majority of the fuel and glow plugs available in the United States are designed around 10-15% nitro.