Mr Brine here....
Brining involves a ratio of salt to water. If you just add water, you'll dilute the solution too much.
I usually make gallon batches and it usually works perfect for a 12 or 13 lb. bird. 2 birds, 2 batches and so on.
My suggestion. If you have a recipe that you like -- double it so you keep the ratio and seasoning levels the same.
Smokin'