That is correct, .16 to .64 fl. oz. per gallon of tank mix. (1 qt per 100 gallons of tank mix)
As for the amount of water to mix with the Ecomazapyr, mix 5 to 30 gallons of water per surface acre. For watershield, since the leaves are floating on top of the water, you don't need to calculate ac/ft of water, just the surface area that you are spraying.
Ideally you want to use enough water to cover the whole area that you are spraying, but not to the point of too much runoff from the leaves. Imazapyr will translocate to other plants nearby if too much is used (i.e. shoreline trees or plants) and will kill or at least shock them. You don't want to over-apply it but at the same time you don't want to under apply it either. Plants will develop a tolerance to some chemicals if they are used in too low of a dosage for a number of treatments - like a vaccine teaches our body to fight off something that could kill us if not vaccinated.
When I am spraying, I will mix in 8-16 ounces of a pond dye to the tank mix so I can see where I sprayed.
You want to treat when the plants are actively growing, treating in the Fall is best for plants that have an extensive root system, but I understand that you want to utilize the pond too. So, just be prepared to possibly treat the plants again in the Fall if they pop back up in late summer. Up here, we like to treat those types of plants in mid to late September. Down there I'm guessing (and this is just a guess) in mid to late October.
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