There are times, especially when working with moulding, that you need to divide an angle in half. I learned how to do this with a compass in high school and since then I have fallen in love with the Angle Finder, an impressive little device that uses more geometry than my compass to measure exact angles easily.
But Ian Burns, the very smart woodworking professor who made most of my props for the TV show this year, taught me the easiest way yet.
Simply take a 1x3 or 1x6, any straight board. Stand it on its edge and draw lines parallel to the two walls. Then lie it down and draw two more lines parallel to the walls. If you draw a line through the intersection of those two sets of lines, you have a line right down the middle of the angle. You won't know what the angle is, but the less math you do, the fewer mistakes you make.
Simply lock off that angle with a sliding bevel and use that as a guide to set the saw angle.