That's good that your SWCG cell isn't showing scaling. That's really the only reason for having the saturation index slightly negative so if the Borates are helping to prevent scale and you wanted to target something closer to zero, then that's fine, but certainly not anything to worry about. A lot of the recommendations we give are guidelines that should be tempered by your own actual experience. For example, the actual TA level that works to keep pH more stable for a pool is different for different pools. Yes, a very low TA will have less carbon dioxide outgassing always, but not every pool needs to get really low before seeing a benefit while other pools (usually those with more aeration) do better with a lower TA. In the same vein, not every SWCG pool needs to have the saturation index be negative to prevent scaling in the SWCG cell, especially if Borates are being used.

And remember not to sweat things if they aren't spot-on perfect in their values. The effects we are talking about for being somewhat out of range are long-term effects and for good plaster not even that much of a big deal anyway. There's a big difference between "oh my gosh, my TA is red so my pH is <4.5 from those darn Trichlor pucks" vs. "my saturation index is -0.5 but my pH is 7.5 or higher". And even if we move the recommended range up for SWCG to 350-450, that doesn't mean you can't use 525 if you wanted to if that got you closer to zero and you didn't see scaling in your SWCG cell. These are guidelines, not absolutes.