PHI used to have us do 180 autos from 500 feet in a 206B during every year during Recurrent. No big deal. The key was to initiate the aft cyclic input first, or at least simultaneously with putting the pitch down. If you put the collective down first, the nose WILL drop and...guess what...the helicopter will start to descend. You'll be at "something less" than 500' as you get stabilized in autorotation. So it's imperative that you get the nose up (not just maintain it at level) which of course also helps slow the decay of the NR. As you make your 180 turn, now it's important maintain a level cabin attitude and not to dive for the runway or pull the nose up further. A level cabin attitude will keep the speed in the ballpark without staring at the gauge. And who cares if you arrive at the flare with 50 or 60 knots - the a/c certainly doesn't.
We did 300-foot 180's too, but they were a wild, nearly aerobatic maneuver that are fun but no one was expected to try in real life.
Having done as many practice 180's as I've done in my career (both before and after PHI), I'm utterly convinced that if the engine really does quit while tootling along downwind at 500' agl, most pilots (probably even me) will land straight ahead and not even try to get it back around into the wind. (I mean, do I *really* remember what's back there?) Thankfully, the 180-degree auto is more of an academic procedure these days, since it derives from a time when helicopters were powered by big, six-cylinder piston engines, spinning at 3200 rpm (yikes!) and that did have a tendency to quit, as opposed to our modern turbines which almost never do unless we run them out of fuel. If I flew an R-44, I'd probably practice them. A lot.