You can read more about various saturation indices and the controversy of whether they really do predict corrosion rates here
. You can calculate the Langelier Saturation Index (actually the Calcite Saturation Index) using The Pool Calculator
. The saturation indices have more to do with whether calcium carbonate, such as found in plaster and grout, will dissolve or will scale, not whether metal will corrode. As you can read from the link, it is not clear whether the "thin film of calcium carbonate scale" really works or not to prevent metal corrosion.
An online calculator for the Ryznar Stability Index is here
. An easy way to calculate it is to first calculate the CSI from The Pool Calculator and use the following formula to calculate the Ryznar index:
Ryznar = 2*pHs - pH = 2*(pH - CSI) - pH = pH - 2*CSI
So this basically says that if you want a Ryznar index of 7.0 or below, then at a pH of 7.5 you want the CSI to be +0.5 which in most pools would be on the edge of forming scale. I wouldn't recommend this. Instead, see what your current CSI is at an if it's negative, adjust the Calcium Hardness up to make the index near zero or very slightly positive (assuming your pH is where you want it near 7.5 and your TA is reasonable).
Low pH is the primary source of metal corrosion as is a high oxidizer level which is especially the case if no Cyanuric Acid (CYA) is used.