We have studied this and have introduced the idea at ASTM meetings to no avail. Far too many variables and variances, plus engineering ideas, seem to overthink this, in my opinion.
Reasons why not!
1. The span to support the pole is different with each manufacturer.
Some measure bigger spans and more of the pole. Some think this is better. They have forgotten it is a SAIL WRAP measurement.
2. Some use a weight in pounds and others in kilograms. (22K does not equal 50 lb.)
3. Some measure from 6" from the butt of the pole and others from 12".
4. Many have had their FLEX NUMBER out in the marketplace for years, and to change or adjust at this point would be devastating to matching future poles to what you are using.
SOLUTION: Very SIMPLE - Only compare flex numbers between the same brand, not other brands. Pay attention here. When you go to the next length pole, there is a new span.
(PLEASE NOTE: Don't sweat the little stuff! The difference between 21.0 and 21.3 is 1 lb. and a 21.0 to a 21.6 is almost 2 lbs)
adjust your grip up or down to average this! (1" = 1lb to 2lb depending on the brand)