No special formula... just math.
If 9X is 100% and you have 8.3" of subtention, then at 3X, which would be 33.3% of the 9X total magnification, you'd have a subtention of 24.9"... so, not perfect, but still, that's pretty close to the listed 25.2".
Using the 8.3" as an accurate reference of subtension, then it progresses as follows (rounded up) for 100 yds.
3X = 24.9"
4X = 18.7"
5X = 15.0"
6X = 12.5"
7X = 10.7"
8X = 09.3
9X = 08.3
You really need to get a large 1 inch grid target and test the subtention values... should be pretty close, but ya' never know, one or several of the magnifications may not be true X.
It's been awhile since I've done this math. It looks right, but if someone sees an error, please provide the correct calculation.