The reason why I say 50 is because the minimum is 40. So you apply enough to reach the 40 mark. Then a few weeks later you test and find that it's in the mid 30's. So you have to get all the stuff together to apply it again. PITA right?? While it might be more work to apply enough to hit the 50 mark, (or higher) it's less work in the long run, especially if you have a flow thru condition.

A battery and an electric trolling motor would save a lot of work..........