Robert,
Your calcium carbonate saturation index is a little on the high side at about +0.4, but usually we don't see scaling until higher than that. Perhaps if the pH went up briefly at times then it could have precipitated some calcium carbonate, but that usually forms on the surface "hard" and not as dust and you'd probably notice the pool being locally cloudy after chlorine addition which it doesn't sound like you saw.
You could lower your TA to 80 or even as low as low as 60 if you wanted to and would be in balance. If your pool tends to rise in pH, then that would reduce that tendency to rise. Your high CH allows you to do this (i.e. have a low TA), so you might as well take advantage of it. Does your pH rise a lot and is it annoying for you? You clearly don't have to do this if it's not a problem for you. If the pH does rise a lot, then do you have aeration features (waterfall, spillover, fountain, shower, etc.)? The process for lowering TA is as follows:
ACTIVITY .......... pH .... TA ... (The following assumes 6.8 is the lowest measurement on the pH test kit)
==================
Acid ..................... - ........ - ... Add enough acid to bring pH down to 7.0
Aeration ............. + ....... 0 ... Aerate until pH rises to 7.2
Acid ..................... - ........ - ... Add enough acid to bring pH down from 7.2 to 7.0 (you may continue to aerate while you do this)
------------------------------------
Aeration & Acid .. 0 ....... - ... Continue this combination (cycling of the two above) until TA is at the target you want
then AFTER you have reached your target TA,
Aeration ............. + ....... 0 ... Aerate until the pH rises to your target pH (say, 7.5).
==================
Net of Above ....... 0 ........ -
Aeration is done using waterfalls, fountains, showers, spillovers, splashing, pointing the returns upwards, using an air compressor with a nozzle that produces tiny bubbles and is put into the deep end, etc.
I calculate that with your starting numbers and your 15,000 gallon pool it will take 64 ounces (8 cups) of Muriatic Acid (31.45% HCl strength) to lower the pH to 7.0. Then it will take a lot of aeration and an additional addition, over time as the pH rises from 7.0 to 7.2, of 128 ounces (16 cups) of acid. After that your TA should be at 70 and you stop adding acid and just let the aeration make the pH rise to 7.5. You should, of course, actually measure the pH and TA and not just trust my numbers -- generally you add half the recommended amount and see where you are at.
If you set your target TA to be 70 ppm, then you can have a target pH of 7.5 and probably keep it there reasonably, or you can have a TA of 60 and a pH target of 7.6 to 7.7
Richard
