The core book says "Henchmen are typically hired for a share of treasure (at least 15% of the employer’s income)"
The Sakkara module, in reference to hirelings "Henchmen... will expect to be paid a 15% share of their employer’s income in addition to their standard wages."
Is this "15% of one share of treasure" number a lower bound on henchman pay, or is it the henches' asking price, or do henches ask for more but typically get bargained down to that number?
If you have just 4 PCs and 1 henchman under that arrangement, then that hench is getting 3.61% (0.15/4.15) of the total haul, while the PCs are each raking in 24.09% (1/4.15). I have a hard time imagining how someone who is probably taking on similar risks and labor to the PCs (also most likely considered expendable, standing between PCs and monsters) could see this as a fair arrangement.
My gut tells me that a half-share (50% of one PC-share) is more believable for a hench who is expected to take real risks in the dungeon. That henchman would get 11.1%, while the PCs would get 22.22% each.
The "in addition to their standard wages" bit is actually pretty significant. ACKS encourages a lot of downtime and travel time between adventures; I've heard that parties tend to go on adventures about once or twice a month on average, and my own experience bears that out. Plus, dungeon runs aren't necessarily profitable, especially at low level; In the game I'm running myself, the party has repeatedly suffered losses and decided to retreat before finding any treasure at all.
The stable retainer is good money, after all - and more importantly, it's good money even when the adventuring party fails to find any good hauls. I imagine at least some henchmen think it's worth it.
I do 50%. It is a) already the amount of XP they are getting (as per p. 114), so we don't have to recalculate this twice, b)is much easier for me to do maths with (i.e. henchmen get 1 share, PCs get 2 shares), c)allows them to build up a bit of nest egg for their own thing (in the game I'm running the players often play their henchmen on sidequests, etc.) and d)the maths are further made easier when they take on henchmen of their own, at 25% of the treasure.