The proportion of calcium to buffer that's needed by every tank will be different. It can take some time to get the ratio of input correct for your inhabitants, but usually parameters will stabilize once you find that sweet spot. As far as the fluctuations you are seeing, auto dosing doesn't ever really stop that. Reef systems are adaptive environments with constantly changing needs. Keep testing and keep adjusting slowly and you will get there. Also, Seachem makes a product in their Aquavitro line called Balance which balances out the carb/bicarb ratios you're fighting. I add the recommended dose after each water change to reset the ratio in my tank and have had much greater stability with it.