We were taught that a constant-attitude auto in an R22 should be done at 40kt, and you'd only do it at night, but it was an academic exercise - I never knew anyone who took one to the ground.
A word about forward speed versus rate of descent in the flare/landing phase. A 22 will autorotate in the 1800fpm area. That's less than 20kt, vertically. It you spear in bum-first without even trying to flare, you've got the skids to absorb energy, then the seat structure.
But you're going forwards at 60kt or so, and all you've got between you and the tree/pylon/house etc is a quarter inch of perspex. Yet people seem to consider the vertical before the horizontal, and prefer to accept some run-on even in a tight area.