Originally Posted by Stu @ M Developments

The screw is what we call a global modifier in as much as it adds a percentage to the entire fuel map.

If your engine is just borderline safe on boost, leaning the CO can and will destroy teh engine when flat out. The correct base position is in teh region of 5 turns clockwise and 2 turns anticlockwise to put you back in teh ballpark, but this is of course dependant on MANY MANY factors such as heads, cams, sensors, fuel pressure etc etc...