Regarding penis measurements:
The only measure that makes any sense is one that measures the amount that would reasonably be expected to be inside a woman's vagina at the critical time. Normally, pubic bone to pubic bone would be considered the limit, and presumably there's enough force in the thrustage to compress any fatty tissue layers between the exposed skin and the hard limit (the pubic bone.)
Given that, the effective measure would be the length along the top to the skin, plus the compression factor of the skin of *both* parties. Given that the intersection of the penis with the planar bound of the opening is likely NOT to be perpendicular to the shaft but angles downward toward the nuts, the actual volume displacement would be somewhat more than if the plane intersection was perpendicular, which would add a bit of the normally unmeasured parts.
But these adjustments are constants, and thusly shouldn't be included in the measurement. The skin compression, though could possibly vary from man to man, assuming the same woman, so that constant should be included in the measurement.