I have a corner joint I need hidden, for which I'd normally use a quarter round molding, but because this is part of a cathedral ceiling, the angle is much narrower than 90 degrees, it's 60 degrees. So, I'd need a "sixth round molding". Is that usually sold in large hardware stores or is this some custom thing?
Took me a couple of minutes to figure out what you were asking, but think I know what your asking. You need a molding to fit against the ridge beam at the top of a cathedral ceiling? Well my friend, you are not going to find one that is not 90 degrees at a box store. I'm afraid the only good solution is to find a friend with a decent table saw and cut a piece of trim to fit. Use a bevel square and a protractor to measure the angle exactly, then depending on the stock you use, either transfer the angle to the saw for a single triangle trim, or divide it in half and do a double back cut to preserve a front detail of the trim. Sorry, not enough time or space to do a complete table saw angle cutting primer for ya, but I think you understand the concept.