The Kodak data sheet (J24) says, when processing in tanks, one litre of dilution B has the capacity to process five 8 x 10 sheets.
The capacity figures quoted above are for tray processing.
One litre of dilution B means includes 30 ml of concentrate, so 6 ml of concentrate have the capacity to process one 8 x 10 sheet.
One 8 x 10 sheet = one 135 - 36 roll = one 120 roll.
So your 6 ml per roll of 120 makes sense.