A screen that is too high or too low can change both airflow and visibility. Windscreen maker National Cycle suggests starting with a clear view over the top edge to the road about 15 metres ahead. That is a more useful reference than measuring a rider while standing beside the motorcycle.
For a measurement, sit on the motorcycle in a normal, relaxed riding position and let the suspension settle under your weight. A helper can identify your sight line and measure the height needed. Two riders of the same standing height can have different body proportions and seating positions, so the suitable screen edge may not be in the same place.
Before ordering, confirm fitment for the motorcycle’s make, model and year. Check where the maker begins its height measurement, too: screens with different mounts or fairings use different reference points. A city commute and a long tour may call for different wind protection, but an unobstructed view remains the first thing to assess.