Hi Marc!
When I gave the "fudge factor" example I wasn't actually referring to aircraft performance, but to certain Rules of The Air. I was trying to say that, for example, the 500'rule was designed with plenty of slack in it so that even if we (accidentally!!) broke it, we would still have plenty of clearance from the person / object / vessel. This is borne out by the fact that we can get exemptions from it, down to (in my case) 200'for filming and the like.
The H/V diagram is an interesting point. I have also heard that it should be remembered that these figures were achieved by a top pilot in favourable conditions. Which seems a bit odd really, and I can't see what the point of that would be. It's not like increasing the safe height of an AOG hover by 100' is going to make much difference to sales is it? So why not put in a fudge factor to help us lesser mortals. Very strange! Any further enlightenments would be gratefully received!