Sir, rewinding a motor is a painstaking job..as i do not have any formula trial and error method is the only way to get it..now i am ok with it...to me a RS500, 380 or a speed400 motors are a rare treassures.so i turned to toy motors that available to me.i simply cuts off the actual winding, and i winds it with more good wires in to specifications.thin wire with more numbers gives me a good performance at low amps.after winding i replaces its flimsy brushes with a pieces cuts out from razor blade and drills to holes o hold graphite tips shaped on am emery paper from starter motors of scooters.and one more job is replacing motors plastic back side ith brass bush. i always took bushes from old scraped taperecorder mechanisms, from its capstan assy, ones diameter matches to the motor shaft...ohh..a job for two or three days...and i always use a current limiter to motors so it will not blows off...