Instead of simply running the siding material all the way up we often use shiplap boards to set off the gables.
Gable end siding shiplap.
A three person team two installers working from staging who feed measurements to another worker at a cut station on the ground can side a large gable end in an hour or two.
Picture of corner trim as installed.
Measure between the trims at the top bottom and middle this will tell you if the ends of your shed side panel are square.
I started installing the shiplap cladding on the gable panel.
Gable end walls exterior is finished with lp smartside prepainted wood panel siding 4 8 sheets tyvek house wrap 7 16 osb attached to the 2 12 s on 16 centers.
The peak is the tricky part multiply half of the overall wall length 12 by the vertical distance from the wall height to the peak 8 to get the gable end s square footage 12 x 8 96 sq.