A: Math is good
B: No idea, sorry!!
By my numbers a Bell 412 at 11900 lbs gross weight and rotor diamter of 46 feet will have a downwash velocity of 38.8 fps or 2330 fpm.
An S76 at 11700 lbs and rotor diameter of 44 feet will have a downwash velocity of 40.2 fps or 2412 fpm.
3090 fpm seems high to me, but I am no guru by any means, just a line pilot.
By the way, the math was given to me by Nick Lappos, I presume it will vary with air density quite considerably. My interest in it was largely to calculate risky rates of descent that might make you catch that downwash and put you in VRS territory.
Cheers, coyote.