curiousone wrote:I had not seen the interplay of pH and hardness discussed anywhere in my research on this issue. Any thoughts on that?

The Insane FAQ that Jeff mentioned above and that I linked in an earlier post is where most of us first learned about limescale and the LSI calculation. The La Marzocco calculator that you used is based on that calculation, but is a little over simplified in its advice. When doing that LSI calculation, the TDS is of very minor importance compared to calcium hardness, carbonate alkalinity, temperature, and pH. Many people estimate the scale potential in their espresso machine boilers using just the total hardness (GH), the alkalinity (KH), and the boiler temperature -- using tables in the insane FAQ. That works reasonably well because the GH (total hardness) is never lower than the calcium hardness and for most water is only slightly higher, and the KH can be used to estimate an appropriate pH for these sorts of calculations.

Also if you are reading your water with a conductivity 'TDS meter' it is only an approximation, and may grossly underestimate your actual mineral content if you use it to test TAC treated water. And if you use a drop titration kit to measure GH and KH on TAC treated water, and use that to estimate scale risk, I think you will get a scale rate estimate that assumes that TAC is ineffective in reducing your limescale risk.