Ralph Lambrecht author of Way Beyond Monochrome has a generous heart and has posted a templates online. There's one for calculating bellows extension and magnification. It's highly accurate and It's simple. Just print out the template, place the square target on your subject and measure fstop compensation with a ruler on the ground glass. I'm not good at math, so I use the template.

http://www.waybeyondmonochrome.com/W...mplatesEd2.pdf

I also recommend buying the book. It's helped me out a bunch. Cheers!