Quote:
The easiest way to get a center to center measurement, which is correct Art, is to measure the group at it greatest dimension and subtract from that measure one bullet diameter.

Unless you are shooting wadcutters the bullet holes are not all that likely to be one caliber to 0.001 inch tolerance.
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tan^1(group size ÷ range) x 60 = MOA...
tan^1 (1.5÷3600) x60=1.4 MOA (yah, yah, 1.4324 etc. but sig figs!)

More than significant figures, an approximation.
You can get the correct answer by dividing the group size by 2, computing the angle,and then doubling it.
MOA is not a perpendicular concept, but a circular measurement.