Mine to is calibrated at .25 of a gal per th
So if your using the low rate of 3 way you just need to add 4.4 oz of 3 way to every gal of water you are adding to the tank.
If your going to fill up a full tank than you need to add 70.4 oz of 3 way.
4.4 oz of 3way times 16 gal of water = 70.4 oz of 3 way
one gal of water will cover 4th 1.1 times 4 = 4.4 oz