I'm guessing you mean X amount of ice equals X amount of water, for some reason I keep thinking a cup of ice will equal a cup of water which you can test to see. But this might work also,if you have some empty bottles you can fill and freeze them and start with ice water in your brine and just add a couple of frozen bottles to help keep the temp where you want it without diluting the brine. It sounds good to me anyway,but I've been known to be wrong.