this is how I square up the corners.
measure 25 1/2 inchs from the dorner along each wall,. cut a board @ 45 degrees on both ends. 36 1/16 inches long to the points. line up the points to the 25 1/2 inch marks and your corner is straight.
this is showing the double 2x6 header beam across the front opening.
above that is the ridge beam sitting in the beam pockets. It is not nailed in. as I go, I can adjust it so that the rafters are square with the walls.
this is taken facing toward the chicken coop