Employ your helper to hold the fixture at an estimated height and then see if it s a comfortable reach for all users.
Shower valve height from floor.
Check local building codes looking up local building codes is a great starting point if you want to install your shower valve at the right height.
I find if you have a shower valve fitted over the bath then 1300mm off the floor is a good height.
Standard height for the shower valve the handle that turns the shower on can be roughly waist high 36 inches above the shower floor or as high as 48 inches.
For shower stalls install the valve at the standard height of 48 inches or 4 feet.
Included in the many details are the requirements for showers and proper shower valve placement.
Height above floor the minimum height for the shower controls area is 38 inches above the floor.
A height of 28 inches or 2 feet 4 inches is standard for valves in bathtub shower units.
If your shower is also a bathtub ensure that the valve is low enough for bathers to use it but you can place it around 45 inches from the ground if the shower does not include a bathtub.