The low ink warning tends to be conservative as you note. The number really depends on too many variables (print made, size, cleaning cycles, test prints, accuracy of sensor, component variances, etc.,) to give an accurate number. I use an old version of the software (3800), and while the printer utility will give you an estimate, but I wouldn't depend on it.
Just have a replacement handy and print until the print cycle physically stops due to low ink, replace the cart and resume printing.