How is the focal length of a lens measured? From film plane to what point of the lens assembly. From what point on the complete camera is the distance to the subject measured. I would like to calibrate the focus scale to the ground glass image on my Speed Graphic. Is this question asked a lot?

In the lab the focal lenght is measured from the "rear nodal point" which is where the image inverts itself. This point usually lies somewhere just behind the lens board.

For practial purposes, use the lensboard to measure out the distances on your scale. 15 feet or 15feet 1/16" won't make that much difference.