RE: how much round up
if you put too much water in your tank, but still apply the recommended amount of gly per acre, you'll have too much water and the solution will run off the leaves/blades.
Imagine spraying 500 gallons of water and 1 qt of gly on an acre. it would be so much that most of the gly would wind up on the soil.
Imagine spraying 1 quart of water and 1 qt of gly on an acre. the coverage and application would be so difficult that most of it would be wasted.
I think 15 gallons per acre is about the right amount.