[Kiske]

For the most part closed or slightly cracked at idle with most setups. You want a bit of air to escape so that when the turbo is still spinning and the throttle blade is closed there isn't positive pressure. Some run it open at idle for faster spooling. Soon as you touch the throttle it should close until release.

If you are running a speed density tune or the BOV is behind the maf then yes, it MUST be closed.

Use your boost gauge to determine the amount of vacuum at idle. Use that to pick the correct spring. Then use preload to fine tune it. The point of having the proper spring in it is to make sure the spring pressure isn't more than what your motor can pull open.

Preload is mainly going to be seen in the car. Free rev it and quickly let off. Watch the RPMS. When you let off do they ever dip down blow idle or does the car stall? If so then adjust it (tighten it.)