It always turns out brown, so just use brown to begin with. Just kidding. Yeah it's drop counting because too much paint stays in the dropper or pipette to measure based on the paint originally picked up. That way, you can keep count of every single drop you put in the mix. The end result... you have the total drops in relation to the drops of each color you put in, and that determines the real percentage.
You use a small amount of paint to find out the percentages, then you can thin all the colors and really use those percentages to make quick mixes of larger quantities of the exact color with marked droppers.