Take 64 (oz. of product that = 50% ABV),
multiply by 2.5 (100/40), and that comes to...
Sub-A: 160 (oz. of total product when all will be said and done (at 40 Proof))
Subtract 128 oz. (Original amount of product) from 160 oz., that comes to...
A: 32 (oz. of water required to be added to the original 128 oz. of product, in order to bring your ABV down to 40% (80 Proof) from 50% (100 Proof.)
BTW...
How many people can tell when their "drink" has been watered and at which point do they get p!55ed enough to tell you that your drinks are "watered down" or just forget the "niceties" and punch you in the schnauze?