The only time I go a little extra rich on the needle is when the engine is on a fun-fly. Hovering and vertical maneuvers generally require a richer high-speed needle.
For my sport airplanes I usually set the high-speed needle a little rich (300-500 RPM). Also, with the help of a friend, I have him/her give the engine full throttle while I firmly hold on to the airplane. Once the engine is running strong I point the airplane straight up. If the engine sags I richen her up a little. I've seen people tune their high speed needles this way with no ill effects.
Make a few trial runs in the air and go from there.
If you're unsure, start a bit too rich and slowly lean it out through flight testing.
Is there an airframe and engine in mind?