The less algae (phytoplankton) in the water, the more P will be suspended in the water for the alum to work on (less tied up in the algae). Lets see what Rex says, I'm more inclined to use 300#/150# per Ac/Ft water.....
Depending on the weather trend (is a warm front coming or a cold front?) I would hit it with Cutrine Plus in the low 50°F range if there was a warming trend coming.
