Why 500ft ? There is no rule saying you can’t come down to just above ground level if no buildings etc
but common sense says you only do full touch down to sites you know and have permission. Personally I have never allowed touch down autos on any of my Helicopters because I think it is a pointless exercise which ends in countless bent machines for no good reason ...I also stipulate this to help my insurance.
if you can get to 5-6ft with no ROD and speed back at 10-15 knots you are home and hosed . I don’t really care about the final touch down as you should walk away from that . When I did my initial training in the States we almost never did full down autos and consequently never damaged anything!!