Could I suggest following my guide and see if that works, if it does it's just a case of editing it to do what you want. If you don't want to do that it would be helpful if you posted your udev rule and your script here so that I can take a look at them. In the mean time I'm going to run some tests to see if my rule is actually working as expected.
Edit: Hey, looks like my rule is faulty too - the data I want doesn't seem to appear in the output of the df command until after the script has exited. I'll post back if I find a solution.