OK, I'm not familiar with the hagen test kit, but I don't think you should count after the initial drop. You should count total drops.
Also, If you want to be more accurate you can count the number of drops it takes to make 20 ml (instead of 5ml) of water change, and now each drop will represent 2.5 ppm instead of 10 ppm.
Also, If you want to be more accurate you can count the number of drops it takes to make 20 ml (instead of 5ml) of water change, and now each drop will represent 2.5 ppm instead of 10 ppm.