There isnt anything to monitor.
Use it till its gone, then mix up more.
If you want to use the full dose, its about 20 drops from the black dropper bottle, it holds about 60 I recall.
I normally use much less, around 7-10.
It is a numbers game
But 1 giardia cyst has infected people in controlled studies,but very low percentage. It was a small study with only a dozen people or so , not that reliable. I think one person got sick. Some didnt get sick with 4 cysts. Even 30% did not get sick with 10 cysts.
This is why it doesnt matter, you have about 1000x giardia reduction in a couple minutes at 4ppm normal treatment concentration. Unless you drink sewage, in the backcountry you are unlikely to get more than a couple cysts, in almost any situation, a small treatement for a very short time works. Viruses and bacteria are eliminated in seconds to 1 minute as well. AM recommendations are based on at least 10,000 fold reduction to be conservative.