I try not to put too much into the RPM and thrust calculations the tachs are a good reference point for tuning, finding max rpm and backing off the needle for proper tune settings other than that if you are running the recomended prop and your plane seems to be bogging through the air try another prop until you find one that gives you the best flyable performance, diamiter does have alot to do with RPM but so does the Pitch of a particular prop and how much thrust the prop produces in the real life situations in a particular airframe.