Lens extension measurement troubles. What to measure?

I cannot 'do maths' at all (as in I can't even add up) but I've been trying to figure out how to measure extension to calculate exposure compensation.
The best formula I found was this :
Bellows extension / Focal length all squared.
By measuring as shown I do get the correct result for 1:1 magnification. Repeating that at a different lens position, it fails. But I'm basically measuring to the end of a lens barrel so maybe the 1:1 result is a total fluke?
I've looked for a chart of Magnifications (as marked on this lens) to exposure compensation factor but can't find one. Threads on the web going into this completely loose me - hence the gravitating to this simple formula. But still too complicated for me. Too confused by it all now. Doing this over a number of days. :(
Help!