Mine is at the 9-10" range. I keep mine wide open and skim dry to prevent excess nutrient loss. This height helps the skimmer to work easier in my opinion.
I attached the data sheet for the skimmer incase you don't have one. It does say the following about the height.
--Put the skimmer in sump and plug it in. Water level in sump should be 6”-10” as long as
it is not covering the water outlet pipe. Turn the gate valve to adjust the water level inside
the skimmer. The water level inside the skimmer should be an inch above where the neck
transition starts for the initial setup.