moopups said:Just because I like to add additional words (?), place a strip of siding in its normal position with the corner touching the underside of the existing gable overhang, temporally secure it. Now place a uniform width strip of any filler material against the underside at the angle of the roof. Strike a pencil line. Transfer this to the material to be cut, it can be reversed to fit the other side.
Be very aware of mis cuts due to travel (vertical factor) and interlocking. This is where we separate the caulkers from the others. If you foul cut a piece just reserve it for the next upper application. But use it to determine the length that can be trimmed. Modern materials are very easy to trim but rather difficult to stretch.