Which volume is used when adding salt additions?

Doing my first all grain brew soon : ) and spent the money on a blichmann breweasy which is a kettle-RIMS system.  This is a “no sparge” system and total water volume is added before dough in.

My question is with a 12 gallon final volume and a 23 gallon total starting volume, which volume do I use to calculate how much gypsum, cal. chloride, phosphoric acid to use?

Since you are most concerned with mash pH, then I would recommend adjusting your water for the 23 gallons as those gallons will be mixed in with your grains which will determine your target mash pH.

thank you  - that is what I figured but I am concerned that after evaporation (3.25 gallons over 90 minute boil) it would leave me with higher concentrations of salts in my final volume than I intend?  Am I missing something with this logic?

If you’re adding salts for flavor (chloride, sulfate, sodium), target the concentration based on post-boil volume. If it’s for mash pH control (calcium, magnesium, bicarbonate), use the mash volume.

Your logic is sound.

I use bru’n water and subtract my boil off from the amount I enter into sparge. This is for batch and fly sparging, so I can’t recommend what to do for a no sparge type system. I started doing this after a 2 gallon batch came out with minerally taste.

Yeah, so long as you’re talking water volume for the mash and not the total mash volume (water + grist).  At least that’s the way it is calculated with Bru’n Water.