I asked my contractor friend how he finished peaks of cathedral ceilings or any other inside corners of angle greater than 90 degrees. He told me about a product called Straitflex. Strait Flex comes in a roll and is a stiff fiber product with a creasing area in its center. So it makes a perfect crease, and because it comes in a roll, it will tape an entire ceiling with a nice straight joint. So I tried some of this myself. You can see my complete trial at Taping Wide Angle Drywall Corners. This product worked really well! A few precautions to take are to pre-fill any gaps at the corner first, make sure to place plenty of mud at the corner, and don’t expect this tape to conform to the wall like paper – it is fairly stiff and if you imbed it too tightly, it will “bounce” back and create a gap. Still, I would highly recommend this product, or something similar for arched ceiling peaks or other wide inside angles.