I'm about 6' 1", using a Cee Bailey +5,+3
Metro riding, <50 MPH, especially in hot weather, I run it almost all the way down to provide better helmet ventilation.
Secondary road riding 45-60 MPH, in run it about half way up, so most air hits my helmet just above the face shield. I get good ventilation, and it's fairly quiet. Good enough to hear the XM radio OK.
Interstate road riding 60-90 MPH, I run it about three fourths of the way up, finding the sweet spot where it's quiet, no buffeting, without much vacuum behind me pushing my helmet ahead.
The only time that I've needed to run it all the way up was in heavy rain, where traffic was running about 40 MPH on the interstate. This kept most of the rain off me, and I could still just barely see over it.