I think you're right on the 24". Have you considered mounting it on a diagonal? That allows for a stander to have a balance point and a sitter to have a place to pull up.

Thank you, Suzannesl. I had not considered mounting it on the diagonal but am hesitant to do so, based on Internet photos. It looks more "hospital"-like to me than a straight horizontal mount does. Do you think it makes much difference?

FWIW, DH and I shower in our master bath (which also has a grab bar).

I recall reading something, some where, last year or so, that recommended you should not put a tub bar on the diagonal as it will not allow you to pull yourself up, because your hand my slip down the slope. I don't know where I read that but it made sense to me. I have a horizontal bar at my tub, similar to the one you show in your pic. I also have a mounted bar at the back of my tub at a high height, to use as a towel bar, thinking that if I fell, I'd want something other than a towel bar to bear my falling weight.

With my new bathroom I have the blocking to mount a long bar along my long wall. It is a 60"x36" shower in an alcove. I was also going to put an 18" vertical bar at the entry but won't now, due to the shower door layout. But I like the idea of a vertical bar at the head of the shower to hold onto while getting into the shower, especially if climbing over a tub.

I just spent a long time going over pictures of grab bar placement for our new tub. I didn't spend a lot of time on the ones on the back (long) wall because we didn't put one there. Like the OP, the grab bar is a "one day it may be useful" thing. In any case, the recommendations for the long wall were almost all diagonal placements to assist users both sitting and standing. Some grab bars come with a textured surface to help with the slippery surface.

In our tub/shower, we were torn between placing a vertical bar at the faucet end of the tub or the foot. Most illustrations (google "grab bar placement") show either one at both ends or faucet end only. We discussed it a bunch and did a survey of whoever we happened to be talking to and finally decided to go for the foot end. The reasoning is that once you get the shower going, you really don't want to enter the tub at the end directly into the spray. So that's where it is. Long decisions for something that seems easy on the face of it.

Another option - if you have a hand held shower bar at the faucet end, you can replace it with a grab/hand-held-shower bar combination. I bought mine from Jaclo. I figured if you slip while showering you are going to try to grab the shower bar, so might as well have one that will work as a grab bar too.