measuring offset
True Offset and Diagonal with 45° fittings
(Click the diagram to toggle the triangles)
 

The formula for the true offset:
true offset formula

The formula for the diagonal:
diagonal formula

Note that the true offset and setback are equal for 45° fittings because it is an isosceles right triangle.

Example
true offset example
Determine the true offset using the square root formula where the offset (run) is 1.5 inches and the rise is 2 inches.
true offset formula