A vacuum occurs behind the screen. When the air flow at the end of the screen meets the vacuum, the air swirls. The larger the screen, the larger the vacuum and the larger the resulting turbulence. There is normally no problem for larger drivers because the turbulence is not around the helmet area. Smaller drivers are therefore better off behind a lower screen. It is a common misunderstanding that the windscreen should force the wind over the driver. This is only possible in seldom cases (e.g. Goldwing). It is ideal for the air to flow lightly off the helmet.
If turbulence occurs then the screen is generally, as described above, too high. To determine the ideal length, one can do the following: During the journey, edge slowly upwards until the turbulence is gone. The distance upwards is to be divided by two. The result is the amount by which the screen must be shortened.
Example: If during the journey, the driver edges up and after 10cm the turbulence clearly subsides, then the screen should be shortened by 5cm.
Source: MRA. Worlds largest Screen Manufacturer (they make screens for Rossi, Lorenzo etc.) and have their own wind tunnel facility.