As Lance says, I'd just draw the panel you want, and then measure a 1" seam allowance parallel to the outline, and draw the larger outline.
If you enter the triangle in SketchUp, you can use the Offset tool to add a 1" offset to your panel. It will also tell you the length of each side, and the area of the triangle... (right click: 'entity info')
Or you can do it analytically, starting with the Cosine rule:
a^2 = b^2 + c^2 - 2b.c. cos(A)
Lance didn't call it that, or express it formally, but that's what he used... As he says, there's a lot of tedious mucking about to construct the corners