The 7880 should be better than even the 3800 on how much ink it uses for cleaning.
Use the job logs to calculate ink usage.
You should also take a checkpoint now, after the initial load, of the CM of paper used and the ML of ink. Then, after a few months, calculate the actual ink usage by ML and the actual CM of paper.
Those two measurements should be close, but might vary a bit. They will get you in the ballpark on usage. Let us know what you find!
I have the ink monitiring turned off on all of my printers - using different, refillable carts - so not much actual detail to share. I use 1.2 ml per square foot for a planning number.
It might be useful to report your results in a standard format like that - per square foot, etc. - for standardization. Or per square meter, etc. Per CM is going to be an awkward number, too many decimals.
Best,
Michael