I have the Schneider 120 Macro Digitar that I use on an ARCA 6x9 FC, and (IIRC) you'll need something like 150mm extension for infinity work and upwards of 250mm for macro work.
Disclaimer: this is purely from my (admittedly rather fuzzy) memory of the last time i used it to shoot product in studio.
Perhaps someone like Ed Edwards can give you a more precise answer.
Matt
Well, quite right, Matt, to shoot and area like 22cm square, the extension is approx 10 to 15 cms, and I can just about get away with using the Sinar standard belows if I don't use a lot of movement. For 1:1 maybe around 25cm. belows extension, though I'm not at the studio now to measure.
By the way, I find the Schneider 120 macro digitar to be the sharpest of all my digitar lenses when wide open and I have two of them, both identical in sharpness. I also have digital 100, 120, 135, 150 180. in either Rodenstock or Schneider, they are all very good really and a tad better than the non digital Apo Schneiders used in the days of film, though people tend to exaggerate a little when they say that the digitars are miles better...