Reset ink cartridge memory to get as much ink as possible. Find the circuit board on your cartridge. Often its brown or green in color with black, white, or gold circuits. Just above the circuit board you will see a small hole. Press a paper clip inside the hole (see image below) and the memory on your cartridge should be reset. Put the cartridge back in your printer and your ink level monitor should now give you a more accurate reading.
Let's face it, ink cartridge manufacturers like to give you "low ink" or "empty" warning messages even when you know that can't be possible. However, there is an interesting hack that may work on some brands that will trick your computer into squeezing every last ounce left in it.