I suggest that you look at the mash in two steps within Bru'n Water also. With your grain bill and 5 gallons of water, see what the appropriate mineral additions should be. Then look at the same grain bill with 8 gallons and see what the appropriate mineral additions should be. Add the difference in the mineral amounts to that second step infusion.
One thing to note with respect to the mash is that the only minerals that really matter in the mash are hardness contributors, alkalinity contributors, and acids. Those are the minerals that a brewer would want to concentrate on adjusting for their mash and any steps.
The relative or total amount of sodium, chloride, or sulfate doesn't really matter in the mash. Those ions can be added in the mash or kettle with relatively equal effect.