Exactly, Nick. I should have been clearer. 15-20% less than the "small tank" time; 5-10% less than the "rotary tank" time. (The rotary tank times are usually, but not always, shorter than the small tank times.) You'll have to fine tune these starting points for your own "system" of doing things.
For a long while I kept track of the "batch" size I ran in the Jobo, to see if the number of rolls simultaneously processed made any difference in the results; it did not seem to matter. This is one of the beauties of rotary processing: consistency. I realize there are disadvantages to it also, compared to, say, stand or semi-stand development.
The main gotcha I've found is that you must make sure you have enough stock solution, regardless of dilution, to adequately develop the number of rolls or sheets you're processing. This can be a problem at higher dilutions. For instance, my Jobo requires about 750-800 mL of developer solution to cover a full (large) film drum. This is as much solution as the chemical holding tanks can physically contain (though the film drums could hold quite a bit more.)
You need at least 100mL of Xtol stock (picking it for example's sake), regardless of dilution, per 120 roll or four 4x5 sheets to supply enough "development" to fully process the roll. So your holding tank can handle 8 rolls' worth of development at 1+0, or 4 rolls at 1+1. Since a full drum holds six rolls of 120 or 12 4x5 sheets (= 3 "rolls"), you are effectively required to process the 120 rolls at 1+0, while you'd be safe doing the 4x5 sheets at 1+1.
