Hi Pierre
Thanks for the added detail. I don't know what to say other than the ink usage still seems a little high to me but may be explained by other factors including quality settings.
According to Epson for photo printing from the XP 850 a Standard capacity cartridge will yield about 145 4"x6" and for High capacity cartridge about 300 4"x6". For colour documents then the page count goes up to 360 and 740 respectively.
I know that Epson may be a little optimistic or using their best case scenario but it seems they are way off in your case!
So printing @ 8"x12" using Standard cartridges should yield approx 36 prints and using High capacity around 75 prints.
Are you printing the monthly calendar as a photo? If so switching to high quality document may save some ink but would require you to do two printing runs rather than use duplex printing.
... I was just wondering if there was a way to colour correct for the 3rd party ink.
...
The only way to colour correct for the 3rd party ink (and your paper) is to have a profile made - 1 per paper type that you use.
Graeme's, idea about CISS may be workable and due to the fact that you will have a large volume of ink the profile should be good for some time. Plus it will help to avoid the repeated cleaning cycles - and, if you are lucky and the ink manufacturer QC good then when you change ink batches there should be no discernible change