Actual ink usage and costs

A question for large-fomat Epson users: At our college, I need a system to allocate ink costs to people who have us print for them on our 9890. Do most of you use the actual ink consumption number that the printer panel reports about each job? Or, do you use an “average” number for ML’s of ink used per square inch or centimeter? Whichever one uses, there’s the question of additional ink consumed for nozzle checks and cleanings. When we compute ink costs to use our smaller 3800 / 3880 printers, we add 20% to cover ink consumed for these functions. Is that a good number for our 9890 as well?

I’m diving into un-replied posts that were buried by our old forum software back in the day.

When I’ve managed edu labs in the past I’ve used PaperCut. It’s great. I use an average $/sq/ft cost. I’ve always doubled the actual ink cost because this covers head cleanings, repair wastage, etc. I think it was at like $3.20/sq/ft with a cheap paper included back in the day but this is just off the top of my head.