For myself I measure my COL from the ogive because of differences in individual billets can vary, and the fact of the matter is that the variable will be the distance that the bullet ogive is from the lands , because that is where the accuracy will be determined .
The amount if bullet past that does not matter as long as the ogive distance to lands is the same and the bullets used are the same.
the COL is then marked on the box.
Cat
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Anytime I figure I've got this long range thing figured out, I just strap into the sling and irons and remind myself that I don't! 
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