If only them digital instruments didnt cause some much we could have a read out with numbers vs. the color. lol
What I did was with a gallon bucket dropped a few drops of ammonia to get different colors of green. I took the sample from the tank and others to get an idea where I was at. Once you do this a few times you will get an idea where you are at. Having 2 test tube of side by side makes it easy to pin point the color. I did it with 3 test tubes just to get an idea. Again when they are side by side it's easy to read where you are at IMHO