I used to shoot a lot of table top shots that required the calculation of bellows extension. I'm dyslexic and can't do math well. Calumet had this device that I used a lot. It was basically two pieces of plastic. One piece was a 2"x2" plastic square where I put in my shot. The other piece of plastic was a ruler about 1"x 4 inches long. I just measured the magnification of the square on the ruler and calculated the bellows extension. It was a simple and slick little device that was effective.

Take a look at this link: