Hi there,
I calculate it as:
Disk loading (in lbs per square foot) X 210 and then take the square root.
This gives the average downwash velocity in feet per second.
By my numbers:
Square root(62170 lbs/8656.8 X 210) = 38.8 feet per second downwash velocity, about 2330 fpm.
Bear in mind, you never fly the thing at empty weight, so you need to provide a realistic weight. The heavier it is, the higher the downwash velocity.