So you think that a water wqith 2ppm Cl and 4ppm SO4, tastes the same as a water with 150ppm Cl and 300ppm SO4? I think the ratio has an effect but only when you are dealing with concentrations of the individual salts that are present at levels that have a significant impact on flavor.
I don't exactly understand the theory that salts are absorbed by the grain so you only count the first runnings. Maybe present in the bound water yes, but the sparge should bring them out just like the sugar so you'd probably get back at least 75-85%. Which I hadn't thought of that so thanks for making thigns more complicated! Yes there are chelators in grain but there are also some salts and we basically ignore those too.
The salts are going to be dissolved into solution. So when the grain absorbs the water, it is absorbing minerals in that water. Even when you sparge, I don't think you're removing water that is absorbed by the grain (I know mine is nice and saturated when I shovel it out of the cooler afterward). When I'm figuring out what is in the kettle, I just take the amount of first runnings and dilute that with the difference between that amount and my final volume. So if my final volume is 11 gallons and I got 5 gallons in first runnings, then I dilute that amount (considering mineral additions) with 6 gallons of whatever I am sparging with (either distilled or my water or a blend) and use the water calculator to figure out what I have preboil. Then I add minerals to get where I want to be.
You put 5 brewers in a room and ask this question, you might get 3 different answers and 4 different methods to accomplish the goal.