you don’t want to be bending too much if you can help it, so go for something just over waist height and you should be fine. as long as it’s comfortable for an hour upwards
That’s a good question - I’ve done my back in from DJing (my headphones-cupped-on-the-ear-via-a-scrunched-up-shoulder technique hasn’t helped me either).
Your back needs to be straight when DJing; for me that’s 110cm (43 inches). I’m 5 foot 11 (180 cm), so maybe about 39-40 inches from the floor?
Having built 3 different platforms for various set ups, I usually go with the control surface being half way between your hip and the bottom of your elbow. This way your hand is always going down to the surface which ergonomically is better with no RSI looming.
Measurements are difficult as everyone is different (I’m 6 foot 5 inches) but that seems to work well for everyone.