Here's Badboy's diagram Morfiend mentioned.

I also made this with Badboy's Bootstrap Calculator. It's at 25% gas, at sea level, with the big gun package in a P-38J. I believe it's reasonably accurate.

Learning for the first time when to use flaps is difficult to do solely from numbers and graphs though. In my opinion the best way to learn, and the way I learned, is to examine films from the 38 gurus. You can watch the needle on the flap gauge to see which flap setting they're at, then observe what kind of turn they're in and what speed they're at. Eventually I watched enough films that I could predict how they would use their flaps, at which point I had an idea how to work my own flaps.
Also, don't be afraid to stick all the flaps out when you need to, especially in a 1v1. Your plane won't fall out of the sky. It's not the best idea to do in a furball if you value your virtual life though.
