charlie,
You ask ", why isn't the vertical band making up the red section of the chart more to the left, or more to the right? What is it that has fixed it at those points? I suspect "testing" is going to be the answer, but I was hoping there would be a mathematical model for it, or a rule of thumb like 50-75% for the descent rate"
Those plot points that are colored red and greenon my website are made from flight test points on a V22 test. They are not where VRS is truly developed, they mark the outer boundary where VRS just starts for a helicopter that is slowing down from forward flight. It could be possible to test from a steady zero knot hover outward, if you wish, and that would scatter some points from the left side toward the center. Those points are not VRS, they are associated with small torque variences (note the percent in the legend). The actual boundary for VRS is inside the oval considerably far from the test points. See the first plot on that page where the orange circle marks the real VRS boundary.
To calculate the downwash speed (which is the reference for VRS for a particular helo) just take the disk loading in pounds per square foot and multiply by 210, then take the square root. That will give you the feet per second for the 1.0 speed value.
To be conservative, if you take a descent at 50 to 75% of that descent rate, you can avoid VRS.
Here is an example For an R-22, disk area 498 sq ft, GW 1370 lbs, disk loading is 2.75 lbs/sq ft. 2.75 x 210= 577. The square root of 577 is 24 ft/sec, which is the average downwash of an R-22 (24 ft/sec is about 14 knots, or about 1440 ft/min).
This means that the R-22 can't get VRS above about 11 knots forward speed (75% of the downwash velocity), but also that it can't get VRS in a descent less than 700 ft/min (50% of the downwash velocity).