I think the calc's are off for the R2880 which has a 11.4 mL cart.
7 * .386 = 2.7 mL
if the R2880 cart is 11.4mL then 2.7 / 11.4 = 0.24 or 24% !
This means that if another cartridge has less than about 1/4 of ink left, then you should change it too !
BUt John, your calcs are based on a different printer and the mass of the ink used.
I looked at the before and after images of the ink level indicator on my printer (the R1900 not the R2880). I estimated that about 1/32 of each ink cart drops during an ink cart change.
therefore:
1/32 * 7 = 0.22 or 22% !
So again this means that if an ink cart has about 1/4 to 1/5 of ink remaining, then you should also swap that cart. Thats for the R1900. Perhaps my newer R2880 uses less. I will have to wait to see when it time to change an empty cart and repeat my estimations again.
So JOhn I think that you're estimate of 1/8 of a cart may be accurate for the R2500, but from my observations and calcs it is being too generous.
Peter