Something to perhaps be respected rather than calculated.
There is a wide scope of variables. The rotor span and all-up-weight of the aircraft would be fundermental as an empty aircraft would create less downwash then a laden aircraft. I am assuming that the answer you seek is goven in windspeed and probably mph rather then knots. The nature of the terrain will influence an accurate answer. The only people who may have sought such information would be the Rotary Wing Test Sqn at Boscombe Down. I doubt even JADTEU who own the HHI course would have access to formal data such as this.
Cranfield considered 'wake' - downwash by another name in performance calculations. Some serious night time reading here:
http://naca.central.cranfield.ac.uk/...rc/cp/1341.pdf