So here’s the deal as to why I asked. In arid climates we tend to play a game with our fill water and pool water where we just let the CH do whatever it wants and then adjust TA and pH to get the CSI where we want. The problem is, when the TA of your fill water is far away from the TA of the pool water, then that adds to the upward pressure on pH rise since you’re always fighting to keep your TA lower than what it naturally wants to be.
So I would say that your “optimal” CH target is one that allows your TA and pH to be as close to your fill water TA and pH. At a CH of 250, you can easily let your TA rise to 100ppm (closer to your natural fill water) and let your pH live between 7.6-7.8, and your CSI will be fine. Then, because you are adding water to your pool that’s chemically very similar to the pool water, the pH should need less adjusting with acid as long as you keep your sources of aeration minimal (aside from letting swimmers have fun).