How to measure the drive shaft..... not "for the ideal driveshaft"

Dana67Dart said
The drive shaft for a 7 1/4 is longer by about 1 1/2"

Look in the parts book for your car and the length and dia are listed for the car, the rear and the trans.

Some years of cars with 7 1/4 rears had a drive shaft with an isolator in the rear. Meaning the rear part of the shaft was small enough to fit inside the front part with rubber between the two

Again, I know what length I need from the specs, what I don't know is measure from where to where. Is it center to center on the u joints, or total length of the shaft, i.e., length A or length X on the drawing. I am also seeing some folks measure from weld to weld (length Z).... for the same shaft those will be three different numbers.