Can anyone explain how this value is determined?
For a regular round pizza with a standard flat profile, the thickness factor, TF, is calculated as follows:
TF = DBW/(3.14159 x R x R),
where DBW is the dough ball weight and R is the radius of the pizza. 3.14159 is the value of Pi (π).
For a rectangular or square pizza, TF = DBW/(L x W), where L is the length and W is the width.
The calculation is far more complicated for a deep-dish style pizza because the dough covers the entire bottom of the pan and part or all of the sides of the pan and there is a small overlap at the juncture of the bottom and sides of the pan. If you are really interested in the math, I think I can find a post to explain that example for you.
You can also read Tom Lehmann's discussion of thickness factor in the article at http://www.pmq.com/mag/2004november_december/lehmann.php
EDIT (1/25/13): Since the link to the above Lehmann article is no longer operative, see the Wayback Machine link to the same article at http://web.archive.org/web/20110404180542/http://pmq.com/mag/2004november_december/lehmann.php