The character I used was a female survivor on the hard difficulty level, in a new game started after installing the 1.33 patch, english version. I raised strength to 60 and agility to 30 to equip the axe (which had a +2 or 3 strength bonus), and did not put any skill points into Augment Damage.
The minimum, maximum, average and standard deviation I listed are all directly from the damage values that scrolled up the screen as I repeatedly killed Jake (3 times without Axe Expertise and 3 times with it maxed, no other changes).
With a listed damage range of 87-137, you would expect an average of 112 damage (assuming a flat damage distribution). With the Axe Expertise damage bonus of 11+50, the damage range should be 98-198, for an expected average of 148.
