Fastrax review and alignment questions for the pros.

I understand the toe in "inches". I can measure that.
But how would it be possible to measure toe in degrees with this type of fixture?
From a quick net search it appears like I have to use toe in inches, tire size and a formula.
So, I have to know the diameter of my tire?
At this point, I'm thinking the alignment shops are guessing numbers to put into some machine to get degrees.