The formulas that I am speaking of are for threading in general. The threads are a standard 60deg thread form, but on a non-standard diameter. Look in the Threading section, under 60deg sharp threads. There are a series of measurements and formulas for calculating the thread depth, major and minor diameters, etc, based on the pitch diameter, which in this case is 1.238". If I had mine handy, I would photograph the page, or give you the page number.
If you don't find it before I get home, I'll post it.
-Cody
EDIT:
The Machinery handbook should also have dimensions for a 5C collet, and various other collets as well. I was just looking at the page the other day, to determine the angle of the taper.