We finally gave up trying to fix the windshield turbulence problem using a home-made mounting, and purchased a V-Strom Windshield Mounting Bracket from Madstad Engineering. It provides a range of height and slant adjustment for the windshield.
The new windshield profile shows it it a couple of inches forward and upward from the home-made brackets. This has pretty much solved the helmet buffeting problem.
The best position is rider-dependent. For me, it is raised up 3/4 of the vertical range and tilted back 2/3 of the tilt range. In my normal riding position, my eyes are 21 inches up and 12 inches back from the gas tank filler cap.
Sometimes DIY isn't better...