Your maths looks ok. You didn't allow anything for diode drop but it is only a small factor.
This equation assumes that the waveform is not a bad approximation to a sine wave and this is generally the case. I have never tried round magnets but I wouldn't have expected much difference.
How accurate were the speed figures for the test coil and how accurate is the new speed?
Was the test coil exactly the same shape as the final ones? if you used a somewhat arbitrary size for the test coil then it may have upset things.
Meter accuracy may be an issue, I have little faith in some of the cheap meters on low voltage ac and this combined with waveform and possibly frequency variation may have had some effect.
I assume you are using a 3 phase bridge, if it is a quick test with a pair of leads connected to a single phase bridge then it will be wrong.
Failing all this just fly it and see what happens. Unless you are in a dreadfully poor area it will perform better over most of the wind speed range. My 10 ft machine with a boost converter comes in at 200 rpm on the main alternator and still starts producing at 8mph ( the output is not as good and a bit choppy compared with the converter). At 10 mph it is fine on the main alternator and I bring the converter off at 12 mph anyway. The converter starts it charging at 6mph but for a direct connected machine you would be foolish to try to get this low as it would really mess up the higher wind performance.
I am sure that cut in at 180 will be perfectly ok but it would have been nice to have a bit more mechanical clearance. I think I would give it a little more and let the cut in go even higher( 1/16" clearance each side wont add much to the speed).
Just watch your furling it may furl a bit higher running faster but it may not actually run faster in higher winds, that depends on total circuit resistance more than cut in speed.
Flux
