Let's try this in SG units. You want 1.053 SG (13 Plato) in 15 gallons of wort.
15 gal * 53 = 795 gravity points.
Now you figure your malt bill out so that it adds to that many points. A pound of Pale 2-row is 36 points. Let's saw brewhouse efficiency is 75%, so that pound is now 27 points.
795 points / 27 per pound= 29.4 pounds of Pale 2 row.
Somehow, we got the same result, but I'm not sure how you did that.
I don't think you should be using the weight of the water at all.