@Angie Gail and
@AliOop -- "Water discounting" suggests there's some official starting point (aka "full water") from which to take the discount.
My recommendation -- Just choose a lye concentration or a water:lye ratio and that's all you need to do. If you do one batch at 33% lye conc and the next at 40% lye conc, then just say that and you will communicate your point clearly. No need to make it more complex than that.
So for example, if you want to soap at 33% lye concentration (aka 2:1 water:lye ratio) for one batch and at 40% (1.5 water:lye ratio) for the next batch, then that's sufficient. No need to talk about "water discount" or "full water".