I am using a bevelled cedar 2x8 to divide the upper wall (cedar shingles) and the lower wall (board and batten). The divider is attached with the 22.5 degree bevelled cut to the wall and is at an angle to shed water as it runs down the shingles away from the lower wall. My question relates to corners. The mitre saw will be set to 2 different angles. The first is 45 degrees to accommodate the 90 degree corner, but what is the other angle? Is there a mathematical formula to figure this out?