Put it where the hose will allow you to reach the back corners of the tub so you can get close when you want to rinse down the thing, but high enough so you can mount it so a tall person can still hold it and reach over their back and rinse off. Keep in mind the drape...you don't want the thing to drape on your tub rim or filler normally when it is mounted on the bar, either. It might take a little play acting in the tub with someone holding the end to see what works out for you. If you have some little ones sitting in the tub, you want to easily get the hand shower around them which minimizes the spray pattern spread. They'd normally be sitting propped up against the end wall.