I came across an excellent article in an old issue of Water Conditioning & Purification magazine entitled "Making Sense of an Incomplete Water Analysis" by Frank DeSilva of ResinTech Inc. This article defines what the minimum required water analyses are for:
Prior to quoting a industrial, commercial, or municipal water treatment system, the first question we ask at Res-Kem is an inlet water analysis. Often we need to work with incomplete or missing information. The main thrust of the article explains how an educated water treatment person can deduce a missing cation/anion value when there are other known water test values available. As a result, you will be able to properly size the water treatment system.
One great piece of information was this chart. It makes it extremely easy to calculate the cation-anion balance and how to size the system. The conversion factor takes the concentration of an ion in mg/liter to ppm as CaCO3.