Different strokes, different folks. Can a tank be successful when dosed by hand? Absolutely! Examples abound! Same goes for tanks with automatic dosing. I find the most success when a tank is stable. And a dosing pump gives me that stability. My doser is set to dose every 10 mins 24 hrs a day. This keeps a constant influx of Alk to offset what is taken up by the corals. As such, when I test in late morning or late afternoon, the levels are very very close. While fish get bigger and corals do grow but we are talking months before any substantial change needs to be made to a dosing schedule. If testing is done regularly, trends can be seen and if you see an upturn in Alk and Ca intake, a simple adjustment to the amount dosed is all that is needed. Water run through a ro/di should be exactly the same every time. Why else would we run it through all those filters!?! 0 TDS is 0 TDS. The only fluctuations that may occur are dissolved gases, higher levels of CO2 can affect PH but that is why we don't chase PH. There ARE a lot of stuff that can influence a tank's chemistry. Which is why we work to minimize those influences. All the changes you give examples of are not drastic changes that would affect how much you dose on a day to day basis. Or even week to week. Any changes will be noted through regular testing and it will be a gradual change which will allow you time to adjust accordingly. Automatic dosing is not the end all to achieving a stable tank. Just like there isn't a fixed number in the parameters that will guarantee you a successful reef tank but a range. Fish and coral are animals and are usually very adaptable. Of course there are some that absolutely has to have their environment at a specific set of paramets but in general, you can run salinity at 1.025 or 1.026 or 1.024, temp can be 74 deg or 77 deg or 80 deg! Like to run your LEDs are 100%, a slow acclimation will get you there. Want to run T5s? MH? 10K? 15k? 20k? All work! But stability is KEY! Getting to your end parameters slowly then keeping them there for as long as you can will usually ensure a successful tank, as long as they stay that way. Automatic dosing does not work for some but they do work for others. If you take note of how many big tank (100g+) owners run dosers or CA reactors vs how many dose manually, I believe you will see a majority automate this part of their maintenance. But again, different stroke, different folks.