There are two considerations, since the 5L kit is chemistry designed for single use without replenishment. One is depending on the specific tank/reels you are using, you need to insure that enough chemistry is present to adequately cover your film for the intended process. Second, the developers have the lowest capacity of all the chemistry so that is the controlling factor. If you want to get into stretching the other chemicals you will need to get into buying individual chemicals and it becomes a bit more complex. I always considered a 120 roll to be approx equal to one 8 x 10 sheet and calculate accordingly. That makes the developer cap approx 1 roll to 140 ml. In my Jobo tanks, I use 200 ml per roll and have no issues. Perhaps I could get away with less, but I don't process enough E-6 anymore for that to be an issue - the chemistry would get too old anyway.
Bob