I've used the circle tempate technique before, worked well for me. Painted overall white, then blue with a big circle, then red with a smaller circle. Here's 2 pics:
This is for a Mustang serving in the CBI piloted by 'Tex' Hill, here's a link to the entire build if interested for more pics. http://waihobbies.wkhc.net/models/P51_HillAVG5/P51HillAVG5.html