This is how i do it.I times the length x the pitch and come as close to the same numbers as possible,heres a couple examples.But the best way is to see how the plane behaves and acts in the air,nothing is better than that.I learned that here.
18x6=108
16x8=128
15x8=120
19x6=114
just an example.If ya want a better answer you did the best thing and ask.Then its just a waiting game for the answer.So if it was me id use a 15x8 3 blade.You picked a 15x7 so your very close or right on.You are right about the screw threads and movement.360 degrees at pitch of 7 equals 7 inch forward movement per 1 revolution of prop.