The "need" to keep the reservoir more than a third full is bs, at least it is as far as valve operation is concerned. As long as the bottom of the reservoir is a foot or so higher than the autovalves, the system will work perfectly until the nute level reaches the outlet to the feed hose. The rate at which the valve will refill pots will slow with lower head, but the rate needed is tiny, so that will not matter at all.
I regularly let my 10 gallon reservoir get nearly empty before bothering to add nutes, and it never failed to properly flood to the expected level in the pots. The bottom of my reservoir was a bit more than a foot above the valves, but that amount of head was still much more than needed to feed three plants through the black ~1/4" tubing.
If you are running multiple plants and using only the ~1/4 black feed tube, you could run into a supply issue but it would be caused by the diameter of the main feed line not nute level, and the problem can be easily fixed by using 1/2" hose until branching off to each autovalve, or just raising the reservoir to increase head. There is no reason to not make use of the all reservoir capacity.
Again, this is relative to function of the valves only - arguments about nute deterioration, plugging, and the like are a separate issue.
Happy autopotting!