Look at the label. It has charts. I don't know if it will work but the label provides that for a 90ppb application on a .25 acre pond with avg. 4 ft. it should take 7.84 ozs.

http://www.sepro.com/documents/SonarAS_Label.pdf

https://www.pondpak.com/ItemGroupItems.aspx?ItemGroupID=38

"Eight onces of Sonar AS Aquatic Herbicide. Treats 1/4 .25 acre assuming average depth of 4-6 feet. "

Duckweed is tough. It often takes several treatments depending on the product. Often one like sonar (to kill the entire water col. including the seed bed in the bottom) and a contact herbicide (Reward) to get the banks and pockets which will pop up.