Oops!

Sean is absolutely right---calcium levels ARE important when you have an SWG even if it's a vinyl pool. I missed that (must have been sleep-posting )

However, I must disagree and say that ONLY when you have an SWG or a heater (gas or heatpump, not solar) is calcium important to a vinyl pool. Otherwise I stand on the 0 to 500ppm as acceptable calcium levels for vinyl.

The SI is really not otherwise a useful tool for pool maintenance. It's actually mis-leading and proper analysis of the basic tests will keep your water clear more effectively. My Cal level is about 70 now--or it was a month ago. I only test 2 or 3 times a summer and ONLY if I'm using Cal-Hypo for some reason.