You put a standard item into your scene, at the distance you are focusing to.

Then you measure the image of the item on your ground glass using a scale that allows you to read the correction factor directly.

Try this link for an example:

and in particular the tool linked thereon (including instructions!):

In case you are wondering, this sort of tool works because you can reduce all the exposure adjustment calculations into formulas that depend on image magnification (instead of focal length and bellows extension).