CSI is "Calcite Saturation Index". It tells you how saturated the water is with calcite, which in turn, lets you know if it can hold anymore.
When CSI is negative, it means the water has the potential to absorb more calcite, and will do so by pulling it from sources which have calcium (ie: your plaster).
When CSI is positive, it means the water is oversaturated with calcite, and has the potential to begin depositing it on surfaces in contact with the water.
Because your calcium is so low, you're hitting negative CSI with a relatively small pH drop. I agree with Pat, and you should raise the CH up, so you can maintain a very slightly positive CSI (while the plaster is curing) with normal range pH. Once the plaster is cured and established, it's best to maintain a slightly negative CSI, to avoid scaling problems.
The plaster manufacturer will have a required CH range, especially during curing, for the warranty as well, and it's most definitely above 50ppm for the minimum.
what would be a reasonable CH ppm for now? 100? 200?
It’s frustrating that I’ve been given zero guidance from my PB and I don’t even know the name of the manufacturer of my pebble/plaster other than “Peerless Pebble” and I can’t find anything on them.