My Own Treehouse of Horror

This weekend I managed to get some solid, yet frustrating work done on the tree house.
The Treehouse.
Before winter set in I was able to get most of the sheathing done. All except for the gable ends. So, that's where I began.
This first side was a piece of cake.
The first side, the front, went easily. A few quick measurements and a track saw make quick work of accurate cuts, even when a single piece of sheathing tucks beneath both roof angles resulting in a complicated shape.
This piece, it didn't fit so well on the 1st, 2nd or 3rd try.
Alas, on the second piece of the rear side (the 4th of 6 total) I measured correctly and then proceeded to cut the piece incorrectly, and then incorrectly again, and again. Finally on the fourth try, I got it right. It turned out, I was measuring using my 26" square. While the square is dead accurate, its important to reference the measurements on the correct side or else the measurements will be off by the width of the square blade.

That's a lesson learned & a piece of T1-11 wasted. Hopefully its now back to accurate cutting.