Um no one on that thread said that you HAVE to keep your tank half full. I said that I preferred it, but if I am trying to drain a tank so I can put it aside or switch out flavors I can and will go lower than that. I just stated I preferred the vape from a fuller tank and that that was my preference.
There is no reason not to go lower, really PRETTY low as much as you can get away with if you are worried about it. You do risk burning your drop in coil, which may or may not be worth say 0.4 ml of juice to you.
However, when the tank starts to get low, you should notice a DECREASE in vapor production, quite often. At that point is the time to stop in my opinion. With that said, should you CONTINUE you will eventually get to the "dry hit." It will probably happen to you, and when it does, you will know it. Think horrendous, burnt awful taste maybe a burning sensation and a coughing fit.
It's unpleasant but easily recognizable, so pay attention when it happens and what your vape production is doing right before hand, as that is the time to discard any and all remaining juice in the tank. You'll be shocked at how little it actually is.
Also, with eliquid, there is a certain amount of waste. You didn't smoke your cigarette filters did you, just because they were a valuable part of the tube you bought, did you?
Think of dry hits like smoking tobacco filters. You can smoke or vape THROUGH the experience should you so desire. But, you really shouldn't.
I think you are obsessing a bit personally, but I remember my own "new vape" obsessions, which is why I bothered to type this all out, hopefully more clearly this time.
Anna