To spray the edges of my pond earlier this spring I put about a half pint in a two gallon sprayer and filled with water. Then sprayed out 2-3' around a 3 acre pond.
It did the trick.
250 square feet isn't much. I would go with a cup in about a gallon of water. If it doesn't kill it in a week, re-treat.
Figuring spot treat rates on any pesticide is a crap shoot.
