I've been trying to figure out why the NED DEMs don't generate a good slope histogram by themselves. Thing is, those DEMs are in NAD83 (I think?) but basically are in a geographic coord system (spherical-ish surface, degrees lat and lon), not a projected coord system (2D surface, meters north and east). That means that a meter at the north end of the DEM is not the same as a meter at the south end of the map - basically you need to project it / save it with a projected coordinate system before you can use a single number like 111120 to convert xy units to z units. But, if you've projected, you're not going to need 111120, you're going to need to use z-factor of feet-per-meter or meter-to-feet as needed or ideally 1.0 if horizontal units and vertical units are the same.
Anyway sounds like your DEM is already projected so that whole sentence about 111120 or 370400 (both of which are inaccurate fudge factors anyway since you need to project it first) aren't relevant.
Note, I'm new at this and have been trying to figure it out for a few days, and got a good answer at this thread and tracked down some more info from there - BUT - please take this with a grain of salt as I still need to verify the slope measurements vs a good ol' USGS topo map.
UPDATE: see the much more clear answers here