Try a lid, it will cut down a lot on your evaporation.
The lid idea is today's (or tomorrow's) test!
But if that doesn't work, how does my logic sound on this:
-collect ~6-7 gal first runnings as usual
-instead of collecting 9 gal sparge to make my 16 gal pre-boil for 60 min boils (or collecting 12 gals sparge for my 90 min boils) collect say 6 more gallons to get ~12 total pre-boil
-at the start of the boil, add whatever additional water I need to get up to 16 (or 19) gallons and boil as usual (although based on my tests, I could probably get away with only 14-15 gal preboil if I reduce the boil rate as I tried yesterday)
A few questions: Would I calculate out the added minerals using Kai's spreadsheet based on the total preboil volume? Which would sort of concentrate the minerals in the mash but they would be diluted in the boil....
Or would I add calculate minerals just based on how much sparge water I am actually using? And in that case, should I add proportional amounts of minerals to the water I'm diluting the boil with, in other words use the same water for everything? To me, that makes the most sense....