To your point about sufficient Ca from the malt, perhaps I should instead be focusing on the SO4:Cl ratio those additions would impart, and not so much the Ca that's coming along for the ride...
Not on the ratio, but on the concentrations of each anion. Add only sulfate and chloride necessary to produce your flavor goals and don't boost calcium needlessly. You can have low calcium in the brewing water. In lagers, almost no calcium is needed and you'll add only enough to supply that chloride and sulfate dose that you want to produce your desired flavor. Having 40 ppm calcium in the mashing water helps with oxalate and beerstone reduction, but its not a requirement. Having at least 50 ppm calcium in the overall brewing water helps ales to clear more quickly, but again its not a requirement.