I have read several papers/discussions on calculating the saturation index. All of them used examples, and all of them chose example test numbers that matched exactly with a number on the table. As you know, the table listings have wide gaps between the number ranges listed. For temperature it starts as 32, the next in the sequence is 37, followed by 46. I have not been able to find anything that describes how to handle the most likely case of when your reading falls between a range? Do you estimate what the corresponding factor might be? Do you choose the closest reading w/o going over? Do you choose the closest reading, even if it is higher? For instance, calcium hardness goes from 100 with a 1.6 index, to 150 with a 1.8 index factor. If your reading is 126, what factor do you use? 1.6, 1.8, or estimate it to be 1.7???

I'd like to see the answer in a formal paper or somethng, but short of that, just knowing the right answer will be helpful.