Bell 427:
You're exactly on the money, like the Flight Manual says. But that's only valid for a no-wind situation.
And sadly, given that you might have put the engineless helicopter into a particular spot, there is no one airspeed that is going to do that for you in all situations. You need to know a lot more than just the 'best glide' or minimum rate of descent airspeed - and that knowing can only be obtained from some experience and practice.