I have that exact skimmer on my 75g. I've had the cup at different heights over the years; right now it's near the lowest it can be set. What you're trying to achieve IMO is skimmate that is not too clear and watery (like lemonade), nor too dark and thick (like coffee). Somewhere between the two (dark green tea) is good. You'll just need to adjust the collection cup until that's what you're getting...takes a little trial and error. And it will change over time, depending on how much waste is in the water column, how efficient the MJ pump is working, and what else you have in the tank producing or removing waste.
HTH,
Mariner