Decided to cut this elephant up into bite size pieces by testing three Canon lenses with five Extension Tube combinations plus "no tube."
HERE'S the link to the results.
Due to the extremely shallow DOF, the focusing point was usually easy to determine. However, manual focusing was usually required.
PrimoFelis, hope this gives you enough empirical data to test your calculations. We appreciate your good work! (Even if some thing this isn't necessary at all )