It's hard to know who to listen to on the 'net, so maybe it'll help to point you to the Kodak Z-manuals. These manuals were originally made for processing labs, and are basically the "user manuals" for all of their color processes.

Here's a link to Z-119, for the E-6 process manuals:

I know it seems a bit complicated, so you may want to go straight to part 5, "Corrective Actions for Processing Solutions". Especially the last page, "Compensating for Low Utilization." This is basically your situation when you process a little, then let the chemicals sit for some time. (You've further complicated yours with "underreplenishment.") Kodak uses a cookbook-style approach here, to figure how much replenishment to use. (Read my prior post if you want an idea of what is happening with the chemical components.)