I do not have any documentation for the older Jobo tanks, but if you will be using it for rotation, with the 2021 reels, you only need the total amount of liquid inside the drum to be higher then the highest point the film will be loaded, which is not much at all. For roll film this might not be the case, and you will need more chemistry to cover the entire surface.
The instructions on our kit only have general info RE re use, and we have only tested one stage re use at + 10% per liter (about 3:45 for dev) and found no significant difference in density. We did not test any further.
For you though this might be worth while testing with some test rolls and see how far the material can be re used before exhaustion.
In short - while the 2021 reels and tank you have might be ideal for pretty much anything, it might not be the ideal tool for C41, with roll film, due to the high volume needed. For that all i can say is the the Jobo multitank system (2500), covers that problem, and gives you exact(and fairly economical) numbers for either 4X5 (2509n reels), 35mm and 120/220 (2502 reels). The cost of buying the equipment should be weighed against the cost of chemistry needed over time with the current tank, but thats just my opinion.
I hope this answers your question.