I'm pretty sure I derived the test correctly; this is how I did it.
chem geek said (according to notes I keep for myself):
1/8 tsp 6% bleach in 1 gallon is 10ppm.
(Also: 1 gallon X% bleach in 10,000 gallons is worth X ppm. But I digress.)
I don't know about you, but I can't measure 1/8 tsp of anything liquid, so I boosted the amounts to something more manageable:
1/2 tsp 6% bleach in 4 gallons is 10ppm. (multiply tsp and gallons by 4; leaves ppm unchanged.)
So, that means if 1/2 tsp of X% bleach in 4 gallons is Y ppm, X = 0.6 Y. (derive by algebra)
How did chem geek get 1/8 tsp per gallon?
There are (4 x 128 x 6 =) 3072 tsp in 4 gallons, or 6144 half-teaspoons.
Therefore 1/2 tsp per 4 gallons dilutes by 1:6000 (close enough).
If your 1/2 tsp is 6% hypochlorite, you have 0.06:6000 hypochlorite, or 0.01:1000, or 10:1,000,000 which is 10ppm. So in this dilution, 6% => 10ppm. (9.8 for the math geeks.)
If you had 12.5% hypochlorite you should see 20ppm. Assuming the bucket and stirring stick were clean. With two of you watching you probably did count drops correctly. (I guess I was being a little too anal in saying you should use a 25ml sample.)
--paulr