Table for DOF with Mamiya 645 extension ring?

[Ulophot]

I am planning a series of still life photos using my 80mm and the #1 auto extension ring, In order to maximize DOF, I'll be using the lens at Infinity, about 21 inches at the plane of sharp focus. My guesstimate looking through the viewfinder is that DOF at f/16 will be roughly 5" in total, 2 in front, 3 behind. Since the prints will be small, I may use f/22. In any case, I'm wondering if there's a DOF scale for this ring with this and/or other lenses, or if someone has the calculation to make. I don't know the added separation distance of the lens from the body and have no way of measuring it.

Thanks.