You have to make a trail and error for the pump. Try it for a couples of days. If foams are going into the cup very rapidly (maybe a few minutes the cup is half full), means your pump is too strong. Lower the pump. If the foam just could'nt get to the cup after a few days, meaning the pump is operating too softly. The liquid should be quite yellowish to brown. You can also indicate by the colour, normally if the skimmer is working correctly, the DOC
collected should not be crystal clear. BTW
, I think for the correct level, the foam should be just at the base of the cup or a little above the base of the cup. If you see foam trying to goes into the cup and drop back down towards the tank/sump water, means you got it correctly. The foams should make several attempts before it goes into the cup.