Safe autorotations at low level are all about speed. If you have height you can gain speed. If you have speed it really does not matter what height you are when you enter an autorotation. 50' would be fine as long as you have forward speed.
A hovering autorotation with no airspeed is a different matter. The Bell 47 is great up to 20' or so, the 206 is better than that.
I presume reference to the "dead zone" is the height/velocity curve. I do wish people wouldn't call it the dead zone. I do lots of autorotations every day in my job and I have never felt that I'm risking death.
Chopperpilot47