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.

JW-- The house is older (builder is long gone) and we did a complete renovation. I have a rather flexible schedule, so we thought we could save some money if I was the GC essentially. Only problem with that plan was that sometimes, regardless of how much you read up, you simply don't know the right questions to ask and you can't foresee any of the million things that could go wrong or not happen exactly as you planned. The tile installer is a good guy. He did a lot of work for us--3 bathrooms (floors, 2 showers, 1 tub) and installed bamboo floor throughout the rest of the house (2800 sqft). Because of the amount of work, he brought in some extra guys. I'm not sure if those guys were quite as experienced, so some things had to be re-done. He's been good in that sense, so far. This bit with the shower tile edge is just something we didn't really expect. He told us he had to "float the wall out" and we asked if it would "look okay." He said, "Sure." Maybe his aesthetics are different than mine. If we'd known more about tile setting, maybe we could have foreseen this result and come up with a solution prior to setting the tile. It sounds like you certainly know what you're doing and, as someone who's had to work with so many different craftsmen and tradesmen in this process, I commend you for taking a proactive approach. It seems like far too often tradesmen want to get started on the job, don't bother to ask any questions, and when something goes wrong the homeowner is to blame. I'm not saying the homeowner is blameless, but sometimes we just don't know what questions need to be asked. We have the answers, we know what we want; we just don't always know the question that needs an answer, especially when it comes to specific trades.

Your fabricators are sadly mistaken - a bullnose CAN be done with a matched leather or textured edge, There are diamond brushes that are available that can be used to apply a matching texture to the stone once it is shaped into the 180 degree return that is known to most people as a "Full Bullnose" edge detail. They can get the brushes that are made by Tenax through Braxton Bragg, GanQuartz or Defusco Supply - and are readily available. this is something I teach at my school - and if I can do it and teach it - it CAN be DONE.... hth kevin

We used bullnose. I really wanted to do custom pencil liner but when we got the quote from our tile place, it blew the budget completely. I thought I would hate it because it didn't match my inspiration backsplash, but honestly it looks complete and I rarely think about it these days. I think either works fine; just depends on your budget and your options.