To the best of my knowledge the only way to get control is to do it indirectly by adjusting the distance between two waypoints, the speed of the aircraft, and the start and finish headings. Indirect but manageable. This is my preferred way to get super smooth pans - set two way points fairly close together with different headings then adjust the pan rate indirectly by adjusting the aircraft speed. You could calculate your rotation rate to hit a target number but finding the optimum with a few test runs is probably the most practical.