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when measuring an MOA I have read (though not from what I consider a reliable source) that you measure to the center of the two bullets furthest away from each other. Is this correct?

Measuring centertocenter of the two furthest bullet holes measures what's commonly referred to as extreme spread. It's one measure of shot dispersion, but not the only one. I and most other shooters use it because it's very easy to calculate. It has some limitations:
If you shoot 3 shot groups and measure extreme spread, you're evaluating that group based on 2 shots (the 2 holes you measure the distance between). If you decide to shoot a 10 a shot group and measure extreme spread, you're still basing load evaluation on 2 shots (but less likely to get a very small group based on random chance as you might w/ just 3 shots).
The fewer shots you take, the more 'useful' measures like extreme spread, that only use 2 data points, are. As the number of shots increase, there are other measures that are better, but not nearly so simple to calculate. You could make the same case for evaluating chronograph data: your velocity range only uses 2 data points (highest and lowest), while standard deviation includes all data points in its calculation.
Wish I knew of a software program that would allow me to point and click my bullet holes on to the target and let the computer calculate mean radius or some other measure of shot dispersion, but until then, I'll keep measuring extreme spread.