You misunderstood the original question. The original post asked about the Ca concentration, not hardness. Calcium at 90 ppm equals 100/40 as ppm CaC03 or about 225 ppm CaCO3. Add to this the Mg ion and your total hardness (assuming a 2:1 Ca:Mg ratio) is approximately 400 ppm or 22dh. This is just one example of why I dont take TOA too seriously. dave.