How do I calculate the takeoff for a 45 degree elbow joint?
First, a definition of "take off":
The length of pipe to be removed (taken off) such that a pipe and a fitting make a specific length.  The term is a concept shorthand used by pipefitters and plumbers.  This is not the same as a material takeoff, which is a list of all materials needed for a project.

Take-offs in general are specific to pipe construction (PVC, copper, etc.) and is a function of diameter.

As suggested by bcworkz, tables for a given diameter and style of pipe are the best method for an accurate measurement.

The formula (listed in your link) .625 x pipe size (don't use the 25.4 factor, unless you are using millimeters) should give you a ballpark for a 45 fitting.