It is hard to define a "Dead Man's Curve" on model helis. I guess it depends on the condition of flight. If you make an intentional auto with enough headspeed, any altitude should be sufficient. If you are forced into an un-intentional auto on the "normal" (low) power setting (barely enough headspeed) then you could be in a world of trouble if you don't have the altitude to maintain the headspeed all the way to the ground.

Rafael