Tom - you are (were) correct with your calculations. I had a temporary brain-bubble. But, as Ted mentioned, residuals will usually remain from a previous treatment for at least a week or so.
As for dosing, the label is simply a guideline (required by EPA). I usually base the dosage on infested areas only (area to be treated), rather than the total water-volume of a pond. This usually reduces the dosage to a more appropriate level for the task at hand - while reducing the associated risks of using copper.
Bear in mind that this same method cannot be used for some other herbicides where the targeted ppm or ppb is essential for efficacy (ie. SONAR).
KD
