Do you calculate mineral additions based on the total water volume used or the final batch volume?

You treat your water based on the total pre-boil volumes. As long as you aren't boiling the crap out of the wort and loosing more than about a gallon per hour, the effect of the mineral additions is correct when they are calculated on the total water volume used in your brewing.

Think about this: In the days before brewers understood that they could alter their local water with mineral additions, they used the mineral content they had and they boiled the wort down as desired. They didn't somehow alter their mineral content to fit the final wort volume.