The original formula was created for closed boiler systems. In hot water systems the pressure as well as temp does effect the saturation of water. The conversation of water to steam will cause an increase of TDS in the water and a drop TDS with in the steam. The primary reason that anticorrosion chemicals were added to antifreeze in automotive cooling systems. We used saturation indexes when setting up cooling systems for large earth moving trucks when I worked as a mechanic years ago.
IMHO
In a normal pool setting these indexes are not as relevant as in closed boiler settings due to high temps and pressure.
Steve