If you want a 90% corner, take two pieces of scrap baseboard roughly 12" long so they lay along the wall nicely. Lay the pieces on opposite sides of the rounded corner, then line the point up. The point of the baseboard is where to measure. The exact same method is used by pro's on any corner to get the exact measurement since even a 90% drywall corner is not truly square.
John L's suggestion would look nice, although be a little harder to fit.
Good Luck, Doug
