With regard to operating a 2-stroke airplane engine, the ideal needle valve setting for full throttle is slightly rich of peak. This will allow the engine to run at peak rpm once it gets in the air and the propeller unloads during flight. Running lean will cause the rpm levels to drop, overheating, and eventually engine wear/damage. I don't know enough about glow helicopters to say this with any authority, but I'd guess that helicopter pilots probably run their engines at peak rpm because helicopter rotors wouldn't likely unload in the air like airplane propellers do.

If you change the nitro content of your fuel, whether it be from 0% to 10% or 10% to 0%, you will need to retune the engine a bit. Additional nitro content will often increase the peak rpms of a two-stroke engine, but the difference isn't always significant.

Your original mix of 80% methanol and 20% castor oil should be ideal for most any modern engine. 25% oil isn't necessary and it will tend to retard your engine performance. Besides which, castor oil is normally more expensive than methanol, so it's cheaper to run 80/20 than 75/25.