Just thought I would voice my experience with using the SI and water balance to prevent calcium scale. Where I live in Southern California, pools can easily build to a CA hardness of 300-400ppm. I have read in more than one training certification manual that by keeping pH, TA in range (pH 7.2-7.6, TA 80-100 non-swg) and the water balanced by calculating the SI and keeping parameters to an outcome of -0.3 to +0.3, that scaling can be avoided even if CA is much higher than 400ppm.
My experience with my own pool, where chemistry and SI are both kept within range is that those statements are false if taken at face value. A typical 20k gal pool usually has areas that are exposed to a great deal of sun, and areas that are mostly shaded from the sun. Even though both pH and TA are kept within range within range, (pH 7.5 or under, TA around 80) and my SI is usually around 0.1, my pool with a calcium hardness level of 359ppm today, and usually around that number, can build significant scale on the sun exposed tile lines and water features within a month! As we all know, temperature is a factor of the SI, and I suspect that a contributing factor is what I will call micro-climates. I suspect that the sun (100+ temps) heats the surface water at theses areas to higher temps than the other areas of the pool, and that the greater evaporation of the water at these areas is a major cause. However, whatever the cause, at least in this area of the country, if you want to suppress calcium scale, you need to keep calcium levels as low as possible, that number being around 200ppm. Our source water here tests at a CA of 122ppm and a pH of 7.6.
So, IMHO, no matter what the books say, in areas where scale is a problem, keeping CA as close to 200ppm as you can will better your chances for avoinding scale. At high CA (300-400ppm) and sunny conditions such as what we have in So Cal and Arizona, you can do everything else right and you will still have scale problems. I have not tried using sequestering agents aimed at helping to prevent scale, but I think they may be worth a try as well.