If you have installed the 1/2 inch plywood and the T 1-11 to the top of the top plate before you put your rafters on you will have to re-figure your rafters as your building is now wider by the thickness of the T1-11 and 1/2 plywood on each side of the building.

The way I figured my rafters is this. If you look at the gable end of a building you will see the rafters and top plate across the front form two triangles back to back. Dealing with one side you have a triangle. You can figure the hypotenuse of the triangle and have the length of your rafter. Formula is A squared + B squared = C squared.

Example a building with a 20 foot span will have a run of 10 feet. 10 feet is the base of the triangle. The pitch is 12/12, 12 X 10 = 120 inches, the vertical leg is 120 inches. So "A" squared is 10 feet so we convert it to inches which = 120 inches "A" squared is 120 X 120= 14,400. "B" squared is 120X120= 14,400. Now add "A" and "B" together = 28,800. Find the square root of 28,800 which = 169.70562 inches long. Take 1/2 the thickness of the ridge board off that measurement and there is the length of your rafter.

Now remember this is the line length of the rafter which is from the tip of the rafter at the top (after the plumb cut) measured down the top edge of the rafter. Make a mark on the top edge of the rafter at 168.955 inches (providing the ridge board is 1 1/2 inches thick) this is where your birds mouth is, take your square and make another plumb mark from that mark all the way to the bottom edge of the rafter, that will be the back edge mark of your birds mouth. Now all you have to do is make the level cut mark of the birds mouth and you have it. Don't forget the 1/2 inch plywood and T1-11 on each side of the building if it is installed before your rafters.

The way I made my level mark of the birds mouth was to take a 2X4 (if the top plate is 2X4) place the 1 1/2 edge against the plumb line you just made for the back line of the birds mouth and slide the 2X4 up the line until the top inside edge of the 2X4 is flush with the bottom rafter edge. In other words, you want your birds mouth level line to be the same width as the top plate. I hope this helps.