Generally, we assume our foe has infinite HP. This is, of course, not the case but it certainly simplifies our calculations.

I have, however, attempted to calculate it and give myself a robust spreadsheet to if I need to change parameters.

I chose a GWF GWM greatsword wielder as my first victim. Because there's nothing like starting with the hardest one first. First up is finding the chances of doing each potential value in your damage range. This is harder for a greatsword as it deals 2d6 damage on a normal hit and, thus, isn't linear Further, and the real pain, is accounting for the rerolls. But I did it! Despite a range (before static bonuses) of 2-24 about 60% of your values will range from 7-10.

Assumptions:

STR 20

hit chance 65%

hit chance post -5/+10 usage 40%

crit chance 5%always use -5/+10

target HP: 100 (roughly CR 5)

All of these are relatively easy to change so if anyone wants different results, I can give them to you.

Foe is a target dummy with a randomly determined HP at time of swing ranging from 1 to its maximum value (in this case 100). This is an attempt to simulate that your target can have variable HP when you attack it and aren't intentionally targeting a foe with high or low HP.

Given the above, our GWM, GWF, STR 20 greatsword user will average 1.16 points of overkill damage per swing, accounting for misses (which, obviously, have no overkill damage). Our attacker averages 9.75 damage per swing so his average damage per swing is reduced by 11.9% to 8.59.

When I get a free minute at work I'll see see how things change simply by removing the usage of the -5/+10 portion of the feat.