i agree that it's difficult to Know the thurst developed yb a prop.
if you did, then P = Thrust*V (V= airplane velocity).
alternatively, P = Torque*rpm.
alternatively, you Could relate the airplane velocity to the propeller advance ratio (i think that's the term, dependent on the pitch setting of the prop, how far does the propeller (and the plane) advance (along the line of flight) as it rotates); so that V = rpm*k (k = ft of forward motion per propeller revolution) and then fudge the units back to ft/sec !