Phil, I don't know the answer to that, but my guess is that no difference could be detected. The whole process doesn't seem to be all that critical as far as I can tell. Various folks have tried various widths from 3/4" to 1" or so, and everything seemed to work fine. possibly the motors ran a little cooler at 3/4" width (assuming a 3" dia rotor).
I'll echo John's experience. After converting about a half-dozen motors, the only thing to strive for is that your flats are the same width, and have a uniform angular spacing between them. As far as precision, I used a 4.5" angle grinder, and they turned out fine using John's rule of thumb given above.
Stop by your local used appliance rebuilder and ask them if you can have a couple of old motors that they pull from dryers when they rebuild them; they make great motors to practice on before you dig into your "good" motor.
Jon
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