I can't help you with your main questions; however....

Quote Originally Posted by pmu View Post
I have to say that I have never seen a tank that can develop 120 roll with 250 ml.
It's still not quite 250 ml, but check out this 120-format developing tank. I've got a couple of these and they use 260 ml of solution (the figure is stamped on the lid, and you can sort of make it out in the auction's photo); however, they can't use inversion agitation, just rotational agitation via a rotating stick. I find this very difficult to get right, so negatives developed this way often have streaks. If you're used to this type of agitation and want to save a little money on chemicals, one of these tanks might be just the thing. I believe that Jobo-style rotary tanks could also do a roll of 120 in a similar amount of solution, but I don't know the exact figures offhand. I've seen posts from people who use Paterson-style tanks in this way (rolling them on a countertop) to save chemicals, but I've never tried that.