If the screw is too loose, there won't be enough pressure on the spring, and ergo the piston will move too easily and may move or remain open during idle. You do not want that.
If it's too tight, then you'll hear flutter from compressor surge during vent conditions because the piston doesn't open quickly or wide enough.
The way I did it was I tightened it down a turn and made sure the idle was good and it wasn't venting. You can actually see the piston through the VTA port. If it's closed, tighten the spring down another 1/2 to 1 turn and drive the car and see if it vents okay under boost (WOT and partial). If it's okay, tighten it down a bit more and check again. Repeat until you just start to hear the flutter, then back the spring off a half turn or so and you should be fine.
edit: Sorry, just reread my post and want to clarify: look through the VTA port. If the piston is moving at idle or open, tighten it down gradually until it closes, and then tighten it just another half a turn more. THEN drive off and see if it vents properly. No compressor surge means you can tighten it down a tiny bit more. When you get just the tiniest bit of surge, back the spring off a bit.
Last edited by iNVAR; 12-10-2012 at 04:29 PM..