The pitch of the thumb hole is determined by the front edge. You can put a straight edge into the hole and it will read the true pitch, regardless of what you have done with bevel. Likewise, the back of the hole does not even enter into the picture! You could open, carve, grind, any shape to the back of the hole you wanted and the true pitch of the hole(determined by the front gripping surface) would not change. You may be changing the shape of the hole, but not the pitch.
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the pooh