I would like to be able to mix random volumes of Iodiophor easily, but I am having a hard time wrapping my head around the 12.5 ppm measurement.

Can anyone tell me where I am going wrong?

As marked on the bottle:

12.5 ppm 1/4 oz per 2.5 gallons

1 tsp per 1.5 gallons

I have a syringe that is marked in mL and a water jug marked in Gallons, so I will be converting to mL/Gallon.

1/4 oz (7.4 mL) in 2.5 gallon = 2.96 mL/gallon

1 tsp (4.9 mL) in 1.5 gallons = 3.3 mL/gallon

Round those two given ratios to 3mL/gallon and that gives 7.92 x 10-4

12.5mL / 1,000,000 mL = 12.5 x 10-6

Are the two concentrations on the bottle not really closer to 8ppm?