Alkalinity changes of less than 0.05dkh in a day is flat out impossible in an SPS system unless you are constantly infusing Alk at EXACTLY the rate is is being used - and even the most sensitive hobbyist test devices can't measure this. A Hanna Checker (what I use) can only get within 0.056dkh - that's 1ppm. Unless you have a several thousand dollar lab-grade spectrometer, you can't measure alkalinity changes beyond 1ppm (0.056dkh).
Stable Alkalinity for me is a fluctuation of 2-3ppm in a day during testing or less (0.17dkh) and less than 8-10ppm over the course of a week (0.5dkh). This means that if you test at the exact same time every day, you will only be off 2 or 3 ppm. If you test at a different time of the day, your numbers can be wildly different, yet still be "stable" because of the way dosing works (if you Dose 2-part or use Kalk and not a Calcium Reactor).
Lets say you have an SPS system that uses 1.5dkh/day of Alkalinity and you have a dosing system that doses 6 times a day. That still means you are going to have a fluctuation of 0.25dkh between doses that are only 4 hours apart. Of course you can raise the number of doses to bring this down, but the more times you dose in a day, the lower each dose will be. And the lower the dose, the less accurate the doser is.
For example, if I test my tank at 1:05pm (right after my doser goes off), I may have an alkalinity of 10.36dkh, but if I test again at 5:00 (5 minutes before the doser goes off again), I may be down to 10.19dkh. However, just waiting 5 more minutes, I will test at exactly 10.36dkh again. This fluctuation throughout the day doesn't mean that your system is not stable though.
