Like Carrelh says..it's not a very exact science. It's a "try & see" sort of thing.

While the .65LA is a pretty good engine, it'll never darken the doorstep of the winners circle at the races. Whatever "speed" you manage to coax from it will surely come at the expense of some other aspect of performance.

I'd prop the plane (SS40) to fly well and leave it at that. If you want a pylon racer....they are available.