You are on the right track, but it is not quite that simple -- diametral increases raise power load faster than pitch increases.

A 12-5 would have a noticeably greater power load than an 11-6, whereas a 12-4 is somewhat closer in power load to an 11-6. You really need to have a tach for application-specific prop selection. What you try to do is prop your partricular application (climb, speed, areobatics etc. ) for the same static rpm as the "ideal" prop (whatever that is), then make an educated correction for airframe-driven effects (fat slow plane vs slim fast one, for example).