I understand the concept of caster and also know that more expensive gauges will do the math. Just trying to do it on the cheap plus understand the science behind it. I don't mind taking the 10 seconds to punch the numbers in a calculator.

The guys who made the caster gauges needed to know the math, I was just wondering if anybody on here knew what the formula was.

Turns out with some more searching I found my answer. It is indeed Camber change from 20 to 20 degrees multiplied by 1.5.

Here are instructions from a camber gauge manufacturer who explains how to arrive at caster using it. If you are interested, skip to page 9.

http://www.solotime.info/dotnetnuke/Portals/0/content/011073_smartcamber_manual.pdf

In my particular case I was doubting my higher than expected results, however turns out some aftermarket parts allow for more caster than stock, hence my numbers.