On common miter saws the most extreme pre-set is a 45-degree angle. When you slide the blade over to the right or left side, it will automatically lock into the pre-set. But there are ways to cut a 55-degree angle by manipulating the saw and the board together. In what's called a "freestyle" method, shims are used against the fence and work piece, allowing the saw to cut a sharper angle.
Slide the blade over on the miter saw and let it lock into the automatic 45 degree pre-set. Lay the pine on the saw against the fence and cut the 45-degree angle on the pine. Now release the lock on the saw that's holding it to the 45 and keep pushing it to the right until it makes contact with the frame of the saw and stops. There is a five-degree slack spot that actually lets the blade go to a 50-degree angle.
Lay the piece of pine back down on the table. Set the protractor for 55 degrees and draw a pencil line across the 50-degree angle that you just cut, highlighting the difference. Now lay the pine back down on the miter saw against the fence.
With the saw off, bring the blade down onto the pine board at the pencil mark. Now ease the angled end of the pine board out from the fence until the angle matches the cutting angle of the blade. When they match, drop a 1/2-inch shim in behind the pine between the fence and the pine, to the left of the blade. Move it back and forth slightly until a solid brace has been established, and the pencil line and the blade are perfectly lined up.
Holding firmly to the pine board and pushing it back against the shim, turn on the saw and bring the blade down, cutting the 55-degree angle.