The course is designed for 0.7 as the optimum height.
Around 0.8-0.9 gives a punchier wave on the weir and fatter eddies for teaching the top end of level 3 skills.
Above 0.9 the course starts to flush out and you get a decent flow of water going down the course.
Special thanks to Daithí Power and Riverspy.net for providing the below information.
Online river level gauges on whitewater rivers in Ireland