I change 20- 25% (100L) per week and my recipe is 1 teaspoon of each baking soda, epsom salts and rock salt per 20L. So every week i add 5 teaspoons with my water change. I have 20L marks on my tank glass so i know how much im changing out. That keeps me on a rock solid Kh and Gh of 11. I dont often test the Ph as it always the same. I used to check every water change now its more like every 6 weeks but always good.
The only time i got small fluctuations was initially when i was fine tuning my recipe. I worked it out in a 10L bucket of my tap water adding a measure of each until i had the hardness where i wanted it. Then i had to fine tune it once it was in the tank with the limestone etc. I'd imagine fluctuation would happen if you were not replacing what you were taking out or just not being accurate enough. My tank is not planted, has no drift wood but has a heap of limestone which could also help.