I don't know a better way, but I do know from looking down from 10 meters it's a hell of a lot more than that
perhaps you could use the condition of the water beneath you and relate it to the Beaufort scale.
Most helos produce something that would equate to a Moderate sea state - that is equivalent to a Force 5 or 19 - 24 Kts
To get to 50 Kts you are talking Storm Force 10 which would give Sea state 8 - if you have ever been out in those conditions at sea, you will know that is not what exists under a hovering helicopter.