Eric,
I have done some drop-size calibration and learned that a "drop" is quite a variable thing. I can't find my notes on the subject just now but seem to recall that I found that there was a range of about 10 to 30 drops per milliliter from the various droppers I checked. Your dropper bottle looks like one of those from BPL and, if so, is one that produces smaller than average drops.

Cooney (1998) in his book "Purification of Wilderness Waters" gives a generally accepted effective dose of 0.2 ml/L for sodium hypochlorite laundry bleach in clear 70 degree water. If I recall, that was about fifteen drops from a bottle similar to the one you showed in your picture. He also recommended doubling the dose for cold or murky water.

You may want to check the amount of bleach you are actually adding. A milliliter is 0.203 teaspoon or close to 5 ml/tsp; good enough for calibration purposes. Also, according to Cooney, sodium hypochlorite is not particularly effective on either Giardia or Cryptosporidium but works reasonably well on bacteria and viruses. It is certainly cheap and readily available.