They're 16" centers. The ceiling is 60" wide which if I went cross ways I would have 36" ends left over. The sides are 42", so I couldn't have used them there. Now that you mention it, though, I could have used two 36" sections on the slanted parts. Lots of extra tape, but I wouldn't be struggling to hang them.The reason I asked is maybe if you ran your rock the other direction on the ceiling you wouldn't have that butt joint you are talking about. Didn't know if the left over rock would fit that bottom wall. If you have 16" centers i can understand you running the rock the way you did though.
Sucks having to do that by yourself. Wish I lived close to you and you can bet your ass i would help you. Plus I slung mud for many years.