Some info.....

At what psi should a set up the Mondo BOV at.....
Part number for SC belt with 6.66/2.75

You don't really set it for a certain PSI. It will open when there is a large pressure difference such as boost on one side and vacuum on the other, like when you let off the gas after WOT. I'm not sure how that particular BOV is adjusted, but with my Turbo XS I just tightened up the spring by shimming it till it produced the sound i was looking for. I probably adjusted it 5 times till I was happy.