[jmbeach79]

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?

[brewinhard]

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. 

[jmbeach79]

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?

[a10t2]

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.

[Stevie]

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.

[brewsumore]

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.

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.