An SM250 is way too big for a 150gal tank. That would be like running a Deltec AP702 or AP703 on a 150gal tank, it's just not sized correctly. I would recommend using a Mini 180, Alpha 170, or Super Marin 200 on your size system.
Having said that, since you've already got it try running the water level right at 9", and don't let the water level fluctuate at all. Try the venturi at 2 turns out, and set the internal water level of the skimmer about 1" under the collection cup bottom. Let it run like this for 1.5 - 2 weeks without touching the skimmer, no adjustments. HTH!