Run some blue painter tape around the perimeter at the same height as the green area's chair rail. After a few days, move the tape higher. Chances are good that using the same height throughout will look like an original design feature.

Yes, 5' is a good height. mine is at 5 and I have just enought space for sconce fixtures.

What do you plan to do above it? framed art? ... the mirror will be too tall if you hang it above the 5' rail. Mirror should be centered eye level... like 60 inches.

Since it's basically all one room, any paneling on the walls should be consistent.

Pictures taken standing in the open door looking into the house would be helpful.

Well done trim needs to be well thought out.

Thank you for your suggestions. This is the view from inside the door.

There's not a decent photo of your foyer so we can sketch anything out for you to view.

Take pictures like the one below that includes part of the floor and part of the ceiling so we can help you decide where to start and end the wainscotting.

Those are beautiful!
Here is the best I could do with the height.

Thanks!

Hopefully this works. I was inspired by the photo. Hopefully this matches the current molding style.

Hopefully this matches the current molding style.

What you have taped out on the wall is exactly what you don't want to do.

The issue with your house is that it's visually all one space and needs to be treated as such.

Having different ceiling heights and the intruding stair demand a compromise.

I'd continue the existing chair rail around the room, painting the green part of the wall either the trim color, or the upper wall color.

Use artwork and a mirror on the entry wall.

The picture I was really wanting is standing in the open door, looking up at the top of the stairs.

Thanks for your help. I also taped out the chair rail. I’ll finish the look with a gallery wall above and continue the green below.
Thanks again for the ideas.

A chair rail in the tall foyer seems a bit skimpy. Have you considered taller wainscotting in the foyer (similar to Beverly’s photos) and replacing the chair rail in adjoining room(s) with either full wall wainscotting or lower-height wainscotting?