If you look in many of the handbooks and online you will mostly find very complicated formulas.  In reality  it is quite simple and just uses basic ratios.    The formula below is for using a steel comparator ball but the math works the same if regardless.

Actual Pipe Wall = (Actual Ball Diameter ÷ Ball Diameter on Film) X Wall Thickness on Film

Notes:

Divide the actual ball diameter or the length of the optical comparator by the measured apparent length of the comparitor on the film. Multiply this answer by the pipe wall thickness as measured on the film.

For accurate measurement the ball or comparator block must be put as near as possible to the exact side of the pipe parallel to the film so that the enlargement of the pipe wall is the same as that of the ball of block.

If the comparator or ball is not touching the pipe because of insulation, aim the source at the center of the space between the two, as shown in the drawing below. This will help insure an equal amount of enlargement to the ball and the pipe wall. (Think of how the source side of an elliptical 2” weld is distorted more than the film side)

Example:

If using a one inch diameter ball and the diameter measured on the film is actually 1.2 inches and the film wall measured on the film is 0.500”, the actual wall thickness is 0.416 inches.

Pipe Wall = ( 1” ÷ 1.2” ) X 0.500” Pipe Wall = 0.83 X 0.500”
Pipe Wall = 0.416”

Below is an Excel spreadsheet calculator that you can use.