Most of the ones I see look very high up, often with the top of the rail higher than the shower glass.
put it so that when its on its highest position the tallest person that will use it will fit under the shower head.
im 187cm and the shower glass is about 195cm and the shower head is slightly higher than the glass
Consider whether you are going to install a showerdome for moisture control. If there is a possibility, don’t have the top fitting above the level of the glass.
We have installed our rain shower point in front wall at 2.1m height on 2.7m stud height tiled walk in shower and the rain shower pipe is 400mm long and we are going to install the top of methven shower rail unit at 2m height which is 800mm long in total so that it can accomodate up to 6 feet 2 inch height people. I am 5 feet and 10 inch.