what is rule of thumb for bathtub sliding door height?

Does anyone have a good rule of thumb for the correct height of sliding glass doors to be mounted on a shower-tub? For example, should the top of the sliding doors be level with the shower head? Or higher, or lower?

My bathroom has 10 foot high ceilings and the top of the tub is 14 inches off the floor. The tub enclosure is tiled to a height of 74 in off the top of the tub. The height of the shower head is 61 in off the top of the tub, while the pipe that feeds it enters the wall a bit higher at 67 in off the top of the tub. Thus, if I use standard (non custom) doors, that usually are in the range of 56-58 in tall (from bottom of bottom track to top of the top rail), the top of the top rail will be below the shower head and a good 50-52 inches from the ceiling. I am worried that it will look too short.

The "rule of thumb" is that the door is 5' high, which makes it about 75" above the floor. IF you were a basketball player, you might want a custom door with a higher bar so you didn't hit your head, but for the average person it would be correct. The height of the ceiling and/or the shower head has nothing to do with it.

Tub doors are 'standard' in the range you mention. I agree it might look a little funny with a 10' ceiling. If you are building for a tall family and rough the shower head some distance higher than the top of the rail, there could be a small problem with water splashing out. You could always use a "shower" door set which will be about 70" tall. A custom door can be had in a price range of less that double a very good 'off the shelf' model. Not that many dollars when we are talking about something you will live with for the next 25 years.