Don't worry about making the mistake. It took waterbear and I a while to figure out that the units were ppm Boron. It took me a year before I figured out that chlorine was measured in ppm Cl2 units. These things aren't obvious until you see them defined somewhere.

4.5 pounds is 2.041 kilograms and 1000 gallons is 3785 liters so that's about 540 mg/L of sodium tetraborate pentahydrate which is 291.29 g/mole. There are four boron that result from this compound with each at 10.812 g/mole so this amount of product produces 80 mg/L Boron or what we call Borates.

I think the EPA report was saying that some products recommended dosages that were too high, but that's not the example given above. As for why some products used such high dosages, one reason is that not all algae are killed at 50 or even 80 ppm borates though most algae are inhibited at that level. Another reason is that higher borates levels provide greater pH buffering.

Current products such as ProTeam Supreme use 2 pounds per 1000 gallons for around 35 ppm Borates which is in their 30-50 recommended range.