Rephrasing my question:
I see three separate ways of calculating the final ppm of various minerals in the end product.
Say I begin with 9 gallons of DH20. After the mash, I lost 1.5 gallons and end up with 7.5 gallons in the boil kettle. After boiling, I lose 1.5 gallons to evaporation and end up with 6 gallons of wort.
Let's pretend I'm aiming for 200ppm of Sulphate. CaSO4 gives 61.5ppm of Ca and 147.4ppm of SO4 for one gram in one water. Therefore, 1.36g/gallon of CaSO4 will result in ~200ppm of SO4 and ~84ppm Ca (200/147.4ppm = ~1.36g CaSO4).
Do I multiply 1.36g by 9 gallons of water - my total starting water? This assumes both that I lost 1.36g*1.5gal to the mash and also 1.36g*1.5gal to evaporation. (Obviously salts don't boil off so this is incorrect)
Do I multiple 1.36g by 7.5 gallons of water - my starting water less that which I lost to the mash? This assumes that by losing 1.5 gallons of water to the mash, I am also losing 1.36g*1.5gal, leaving me with 1.36g*6gal CaSO4/gal.
Do I multiply 1.36g by 6 gallons of water - my starting water less that which I lost to the mash less that which I lost to evaporation? This assumes that the 1.5gal I lost to the mash did not also lose the 1.36g*1.5gal CaSO4, leaving me with 1.36g*6gal CaSO4/gal.
I'm reading Brewing Better Beer and Gordon Strong states "Homebrewers should be aware that you don't have to put all the salts in the mash, and that not all salts in the mash carry over to the kettle." He doesn't however make explicit that which I am asking -- namely if I lose X gallons in the mash do I precisely lose the ratio of salt to X gallons as well.