The numbers should be the same. The surface areas of 135-36 and 120 are the same.
It has to do with how much you have to pour in to cover a whole roll in the cylinder. 4 120 reels will be about as high as 5-1/2 35mm. For instance, a Patterson System 4 Multi-Unit 2 Tank will hold 5 35mm reels, but only 3 of 120. It makes a big difference if you use once-and-dump developers. You are correct if you are talking about replenishing or when using something like regular t-max developer that is made to be re-used, where all that matters is surface area.
That's all right, Leigh. This was driven into my head last week when I measured chemicals for 2 35mm and 1 120 reels, forgetting that it actually had 3 120's. The top half of all the pics on one roll are really weird looking.
That's exactly what I was thinking. My Paterson tanks need 500 mL per 120 roll and 290 mL (I round to 300) for 35mm. Since I never re-use film developer - I make dilute working solutions - I make what I need and I dump it after. A litre of D-76/ID-11 stock will give me 4 rolls worth of 1:1 developer for 120 at 500 mL per roll, and 6 rolls worth at 300 mL per, with 200 mL of stock remaining in the latter case.
I guess the point I was trying to make is that D-76/ID-11 is a lot cheaper than Ilfosol, it's very good, and it's quite convenient. The mixing isn't a huge problem since you can mix up pretty modest amounts of stock solution at a time. (It's cheaper if you buy 3.8L or 5L packages, but you have to go through it faster, of course. It only keeps about half a year in full, closed bottles.)
Jim MacKenzie - Regina, Saskatchewan, Canada
A bunch of Nikons; Feds, Zorkis and a Kiev; Pentax 67-II (inherited from my deceased father-in-law); Bronica SQ-A; and a nice Shen Hao 4x5 field camera with 3 decent lenses that needs to be taken outside more. Oh, and as of mid-2012, one of those bodies we don't talk about here.