I would go with an 18'' to 24'' extension , depending of the users height. Some will go with a more appealing aesthetic look , closer to the ceiling ........ but you can always change the extension and see how it does accommodate your needs .

How big is the rain head? I like the idea of it being about 8"-18" down off the ceiling. A small rain head 24" off the ceiling might look odd. A 20" rain head 24" off the ceiling would look good. Scale and Balance I think....

"I'm not sure if gravity would raise the pressure actually..."

I think if this was an issue there would be Rain Warnings keeping us inside until the "Killer Drops" passed

well I think the idea was that it's possible the maximum water pressure you'll receive is directly from the shower-head. Once the water leaves the shower head the droplets get decelerated by air resistance, and accelerated by gravity.. which is stronger- I'm not sure. (So really the question is , is whether there is a noticeable increase/decrease is pressure the higher the head gets hung)

I think I'll be going with a 13" rain-shower head. It's a pretty small shower, bathtub conversion.

I think a 12" extension is the winner here with that size rain head.

The water pressure is not anything to worry about. Silly really people even thinking that this is a danger or something that will cause discomfort. It's water. restricted to 2.5 gallons per minute spread over 50 plus outlets and falling less than 4'.

You might read this story as well for some more pointers

[HOW TO PLACE SHOWER CONTROLS FOR BATHING BLISS[(https://www.houzz.com/magazine/how-to-place-shower-controls-for-bathing-bliss-stsetivw-vs~4690224)

Thanks for the comments! Just to clarify, I am on the side of worrying the pressure will be worse. (since the air resistance may slow down the initial pressure being shot out of the shower head). Sounds like a non-issue either way though.

Great Looking Shower Tundra.

I have a hansgrohe rain head that is wall mounted at the fixtures recommended height of 7'. I would think your fixture would have a recommended height. The issue I notice with my rain shower is that the water feels cooler then my handheld, presumably because of cooling as it travels the extra distance compared to the handheld. Those droplets cool off. I have to adjust my temp when I switch between functions. If you were to place a different control for the handheld and the rain head that would eliminate the temp issue. It is a minor issue, but does exist for me.

It's a tricky one, we have a client with 3m (10') ceiling, the shower arm only comes in 450mm (18") max length. I hope it will be ok, the supplier doesn't specify the recommended distance.

Can I ask what you ended up installing? I also have a 9' ceiling and a 12" diameter rainhead. We are installing over a free-standing bathtub, so we'll have a 360 shower curtain that will drape around it. I have a 12" arm installed now, but am worried that with the water pressure it'll barely get soap off of us (I am 5'7" and my hub is 6'). I hadn't planned to install a 24"...but I also don't want the pressure to be so poor we can't rinse off.

Dragonflygirl girl, in my experience, water pressure in the water coming out of the pipe no longer has much influence on how ”hard” the water falls in a rainhead. My thinking is that the water ”uses up” some of the kinetic energy it gained from being under pressure in the pipe in the effort to spread out in the 12” diameter rain head. I don’t think having the water in the pipe until it is closer to your head matters much for pressure. In cold weather, though, I think that it does have a noticeable influence on the temperature of the water that hits your body. I might seriously consider a longer down pipe.

Regarding the pressure loss. That is a physics problem and I'm a Mechanical Engineer. I think the pressure drop form the extra foot of height difference is negligible. 1 psi = 27" of water column so the extra foot of height will drop the pressure by 12/27 = 0.44 psi. Most water supply pressures are between 30 and 60 psi so you won't see much difference if it drops less than a 1/2 psi. Your water pressure probably varies more than that on a day to day basis. I would target 7' or 84" since that's the common recommendation but I would prioritize how it looks if that is too long.