Here is a link to the chart I use. It has been adjusted for the inaccuracies others have pointed out unless you have abnormally high phosphate levels and/or other buffers in which case it wont work. The same inaccuracy existes for CO2 test kits i.e. they can be thrown off by buffers. Also, a drop checker can only tell you if you are Low, OK or High - not where you are within each range which can vary considerablely.