16-18 good safe conservative level that allows for all situations . Running higher boost with wound back timing is something a good tuner wont do its a waste of time.

A boost level with timing that provides good AFR and no knock will drive 100 times better then turn the boost up and keep winding timing back to control Knock.

You should be chasing drive ability and a nice big consistent hump on the dyno sheet the more space below the line on the sheet nice and wide . Chasing numbers or certain boost pressures is never a good way to tune .