I always maintained electrolyte level -between- the top of the plates and the cell fill cylinder.
It is sooooooooooo easy to check the inside of the cells during max charge rate and see just how vigorous (or not) bubbling is. Proper intensity of electrolyte bubbling does not mean a roiling boil. It means 2-3 small bubbles per second. Hydrogen and oxygen atoms produced by electrolysis not heat.
Correct determination of sufficient battery charge is determined solely by the density of electrolyte, not degree of electrolysis. Specific density of electrolyte. Antimony migration can increase electrolysis 500+% A positive indicator of end-of-life cycle.