The general practice for tuning a pipe is to start with a long header and cut it down 1/4-1/2" at a time with peaking the engine each increment until you see no gain in rpm between two increments. But it's been said that this sometimes causes the pipe to run a little short in the air. So I think it would be good practice to run a prop with maybe 1" more diameter and same pitch as the intended flight prop and tune the pipe to the larger prop and then for flight swap to your 1" shorter prop. This way your pipe ends up being a little longer.
This might be of some help - Dub Jett has some how-to videos on YouTube now with tuning a pipe being one of them. Also - in one of his other videos, he shows a different way of finding your peak rpm that's different than just peaking the needle. It might be worth watching that video also. Here's the pipe tuning video link -
https://youtu.be/KCIlP6_Z0pw