I removed some ceiling beams and as part of the process needed to remove some trim. I cut a board to size and put it up to test the fit. Fit is great except there is a gap as something is not quite level I'm guessing the ceiling. I attached a pic. How can I make the board flush through the entire span. It is 13' stretch. I'm not concerned with where the trim meets the ceiling as I wil put some crown molding there but I want the trim to be flush with the board that it meets at a 90 degree angle. You can see in the picture that in the middle there is a gap.