Ahh! Scratch what I said above... happy2.gif
I get what you're saying - you're actually subtracting the wrong thing. Don't subtract the number of .01 drops you've used. After the color changes, flip the syringe so that the tip is at the top, look at the number the black ring stopper is at, and then use that number to subtract from 1.
You said you used .19ml, so (1 - 0.19) * 500 = 405 ppm of calc