Try looking in the flight manual, the manufactures spend a lot of time and money working it out for you !
H 500 has put it in a nutshell. The manufactuer gives you the speeds to keep the blades turning fast enough to enable one to carry out a landing without power. Wind has nothing to do with an auto apart from the fact of where you are going to land.
Remember Newtons law? For every action there is an equal and oppisite reaction. Pretty much means you cannot get something for nothing.