I think the question is not how to make the return but where to align it. Most mitered returns on a chair rail happen with the bottom of the chair rail aligned on the outside of the last tile, not the top, so the chair rail return ends up protruding equally in both perpendicular planes. If you can't interrupt the quarter round to run the return then I think your solution of scribing a small piece to fill the gap will work the best. It will look sort of like when crown moulding returns on itself short of an outside corner that it doesn't want to turn on a wall. In fact, I might be tempted to return it a bit short of the corner if you're going to do that and not butt right up against the quarter round so it looks more planned and intentional and less like it's just slamming into the quarter round corner. I would align either the top or the bottom of the chair rail (and play around with it to see which looks best) with that last grout joint you have there. That would mean cutting the top of your field tile though and with those rounded edges it might not looks so great. It's something I'd have to try a few different ways to see which looks the best to me.

it's simple. The end piece of chair rail would get mitered as if you were going around an outside corner. Then a "plug" would be made from a scrap of mitered chair rail, and the finish product as shown the the pic below:

I messed up and bought a non-vitreous, unglazed chair rail for use in the shower. The description says it is travertine. My husband is installing it DIY and has installed about 3/4 of it. It runs around the shower perimeter at 61" from the floor. My husband is very handy, but this is his first tile job and it is taking him a long time. Shall I brave peril, ask him to take it down, and try to find something else? Or will I be OK with annual sealing? Do I seal before grouting? If the chair rail's OK, can I use a 2x2 of another non-vitreous tile for the niches (back, sides, top & bottom? It looks like the same stone but the description does not say it's travertine. The niches are on the wall opposite the showerhead, 39" and 65" from the floor. The chair rail at Lowes.com The potential niche tile at Lowes.com Thank you!!

I'm with GreenDesigns, tile the wall. I would think you could notch out the cove piece if you need to, to have the feet fit the coved area, and the back of the cabinet flush to the wall. It's easy to drill through tile, but do excellent planning before the wall board and tile go up so you have mounting blocks all across the vanity length. That's what I would do, then I wouldn't worry about hitting a stud for the mounting. If you ever have to change out the vanity you will have options when the wall is tiled completely.