Ph could come up to 8.3. A buffer, like Seachem super buffer, can help. I'm not sure what system you are measuring your KH with. My test reads in drops, being 9-11 drops is the target, each drop representing 1 degreee of hardness. Then you can multiply them out by 17.9 mg/l or ppm. In short my test kits target 180ppm as ideal. I've seen another kit only once that measures in fractions like yours, I thinik it was a RedSea kit. And if I remember correctly 5 was the target number, if so your 125 is right on.

I'd think that a few 25% water changes should put your levels spot on as your salt mix should contain the right levels. And since your tank doesn't have any demands upon it I don't see why water changes wouldn't put you at your target until your demand becomes higher. Of course this is dependent upon your selected salt mix. Since you don't mind doing tests you should test a fresh batch of salt mix and see what the levels are straight from the bucket..