I am not a big csi fan. It can be a helpful tool but it is also irrelevant in most cases.
CSI measures the "tendancy" for a pool to precipitate calcium. It doesn't mean you will, it means you are in a zone where you may need some adjustments to prevent calcium scale.
The caveat is two-fold....
1. If you have a vinyl liner pool, it can most always be ignored.
2. Importantly, if you stay within the guidelines we test for at TFP, you can ignore csi once again.
Certainly, there can be a precious few outliers wherein CSI gives you a better picture of your pool water management. Many people like to use it as a backup and there is certainly nothing wrong with that.
Do the calcs for it if you wish but staying within the parameters set forth in "The ABC's of Pool Water Chemistry" up in Pool School, will accomplish almost the same thing.