All three of the Epson printers I have used have a graphic showing the approximate levels of ink remaining.
Mirage has a graphic with a percentage reading as well. I don't know how accurate it is, but I do know that it seems to read in 1% increments, given that one of my cartridges dropped from 4% to 3% yesterday after I finished printing (with Qimage).
The accuracy is only going to be as good as the hardware is. Mirage could indicate fractional values, but if the hardware only actually measures in larger increments, well…..
To my (limited) knowledge, most ink ‘readings’ are approximation, which is why Epson has ink remaining in cart when ‘empty’. Canon Pro carts are actually able to go empty as they have a reservoir.