When I mix my nutrients I start by filling my container with water, then I add HydroGuard (to prevent root rot) and mix well. Next, I add my Cal-Mag and then mix well again. Then, I add my base nutrients and any other micro nutrients and give it one last shake. Then I check the pH and adjust as needed from there.
When I top off the reservoir during the week in between changes, I was just using pHed HydroGuard water but this last time I tried adding everything but the micro nutrients and topped the reservoirs off to see if that helps or hurts them. My thinking is that if you added only water to the mix, you are thinning out your nutrient mix and dropping the PPMs but by adding more of what was already in it, you change nothing.