See the very bottom of the page. Click link. What Ralph and some of us use is a variant of this in which the target is square instead of round. At one time Calumet sold such a tool but there are several available on the internet.
Please see the attached for another tool to workout bellows extension
this one was for 35mm but the idea will work with any format
But thanks, all those pages are too complicated for me, I looked at it and was like, "yea no.." Lol
I already use the reciprocity timer app for all my work, but just in case I'm ever in a situation where my phone doesn't work (like a week in the canyon) I would like to have some kind of card to help out :)
In fact, I don't even know if I'm measuring right, I assume it's from the film plane, to the center of the lens (lens board area) but it could be from the rear element? I don't even know haha, I'm not very precise with it, but it hasn't seemed to affect me too much.