Rayb,
It depends on what type of plant you are trying to kill, the total area (& possibly volume if you're after a submerged species) of water actually targetted, and the rate at which your sprayer discharges. The last portion is important because you don't want to get half way around your pond and realize that your chemical is gone. In this instance, you would have over-applied if you calculated the mix to make it around the entire pond.
Be sure to read the label because it will tell you the recommended dosage per plant per acre-ft.
If I'm still up when you reply, I'll try to help you out. Otherwise, I'm sure others will chime-in to help you.
