the coyote,
You are exactly on. The disk loading of the R-22 is very light, so it does not need to push the air very hard to get its lift. The downwash velocity of 1440 is correct. The .002378 is the density of air at sea level. The density is important because the formula simply matches the downward air momentum change needed to make the upward lift for the rotor. With denser air, less velocity is needed, and vice versa.
OK, so we estimate that you get into the beginning of VRS at 50% of the downwash speed. That means by about 700 fpm ROD you should start to get VRS effects. You have observed that at 62% of that speed, 900 fpm, you are fully into the effect (that is the orange area on the plot on my website.) It all checks very nicely.
I think it all fits. The misleading thing about my post is that I have assumed downwash velocities more in the range where turbine helicopters are built, note the use of a Jetranger for the website chart. I will edit the post right now to add Robbie numbers! I will also put a Robbie scale on the chart right now.
Turbines have higher disk loading bcause they have more power, so they can afford to spend more making higher downwash velocity. I will eventually add that to the web site, too.