I'm with you Don,Don,
Not sure I have a direct answer to your question but...
For me, skimming works best when the water level is close to the bottom of the skimmer mouth.. It works worst when the water is at the top of the skimmer mouth.
The problem is that if you run near the bottom of your skimmer mouth, you run the risk of the water dropping too low and the skimmer running dry.
That is why the middle of the skimmer is recommended.. A balance between good and evil..
As long as you watch the water level, or have an auto fill, then in my mind lower is better for skimming.
Seems to me that since all skimmers are not created equal, that this is one of those situations where the pool owner just has to experiment with it, and adjust the level to suit their specific pool.
Thanks,
Jim R.
I used to keep my levels halfway or above. However, after observation, I noticed I collect much more debris when my level is 1/4 ish. I think it sucks air every now and then but it's so little am I really doing that much damage to my pump? My skimmer is a 3/4 hp single speed which I just had the bearing and seal replaced. I like having my skimmer do most of the skimming! Like you said, every pool and skimmer is different.
Bob