Rotary processing in a Jobo processor with the 1540 tank will need 470ml of chemistry, they are giving you 10ml to play with. I generally use that tank combination to process 4 x 135 or 4 x 120 rolls of film. The alternative is to develop 2 rolls at a time using inversion (the method you are currently using) using a bit under 500ml. Whatever the amount is I round up to 500ml in both cases. Two weeks ago using inversion, I did two rolls of 135 in a Jobo 1520 tank using 500ml of developer. Yesterday I developed two rolls of 135 using rotary in the same tank, using 250ml of developer.
To answer your question, I would develop two rolls at a time via inversion for the 135 or 120 as my Jobo 1520 tank can safely do 2 x 135 and 2 x 120 rolls at once. Then I would use the same chemistry to develop the other two rolls in the time it takes to clean and dry the reels and tank. I have done this in the past quite successfully.
C41 or E6?
Mick.