Fit One Miter at a Time
Whether you're edge-banding a tabletop as we're showing here, trimming out a window or door, or installing baseboard, it's always best to fit one miter at a time whenever possible. Start with a scrap of molding with a miter cut on it as a test piece. When you have the first miter fitting perfectly, mark the next one. Then cut and fit the adjoining miter before you nail either piece. For edge banding, work your way around the project using the same process for each edge piece.
Guess and Test
There are all kinds of ways to find odd angles, but most carpenters simply make a guess and then cut a pair of test pieces to see how lucky they are. The angle of these two walls looks to be less than 45 degrees. A good guess would be about 30 degrees. Divide 30 by two to arrive at the miter angle, and cut a couple of scraps at 15 degrees. Here there's a gap in front, so we need to increase the angle slightly and recut the scraps at 16 degrees. When you've zeroed in on the correct angle, the scraps will fit perfectly, and you can then cut the actual moldings.