On one of my yearly tests to see if I still had what it takes, a very well respected and ultra high time instructor Ex RAF showed me that 80% limit in the Robinson R22
It SCARED THE POOP OUT OF ME 
It is so slow you can nearly count the revs with your eyes, everything is at that point going wrong if any one gets near to or past that point you should not be flying in the first place, in fact if you go past that point you wont be flying you'll be dropping "Like a Stone"
Read about it and understand it, Dont go There EVER or you will die!!
Peter R-B
Vfr