Did the test - all 3 settings on same bit of M4 in both directions within 2 hours. Windy and moderate traffic. Results:
Low - most wind noise even at low speed but least sudden bike movement at high speed.
Mid - wind noise a lot less at lower speeds and slightly more bike movement at high speed.
High - very quiet up to 50 mph. Over 70 mph bike most likely to be unsteady occasionally with sudden gusts.
Decided to stick with high setting as majority of my riding is sub 70 mph and not motorway. If I did motorway miles a lot I'd probably go for a touring screen but for my riding I prefer the look of the stock one. Most of the time round and about the standard screen should serve me well. So for my 6' height, it's the high position. I did not like the wind on my helmet as soon as I pulled off, on the low position, more like a naked bike.
One of the reasons I bought the DL was to stop feeling I had to break speed records on the Daytona. If I can learn to enjoy a more restrained motorcycle lifestyle, that will suit this 65 year old!
Tip - when re-fitting the screen, put the grommets back in, push screen with them on just into the fairing holes then press the bolts in to push the grommets into position before tightening. The rubber pushes in easily from the centre with the bolt.