The problem causing the offset is that your diagonal is not centered to the bisecting line of the two brown triangles. You need to measure how long your brown triangles need to be on a flat side (not the center diagonal side). Then you want to subtract the distance from the edge of the orange strip piece to its center, along a 45 degree angle. Cut a large square of your brown material with the measurements you just calculated. Then cut it in half along both diagonals. Sew your orange strip to both quarters in each half, leaving enough extra in the corner to square it (the orange strip will end with a point in the corner and the brown pieces will not go to the corner). Then they'll match in the middle, unless the quilt back isn't square. The offset will still be there if that back isn't square because the angle of the bisectors (of the triangles) will be flipped (on the other side) with respect to the diagonal from corner to corner. If you draw a line from the corners, your orange strips are the same distance off the diagonal, just on different sides.

Sorry that was so long. I hope it made a little sense!

Diagonal piecing is only efficient if your quilt is less than 50% wider than your backing. any more than that, and you are better off with straight piecing. So a 60" quilt, for 40" backing. In fact, I would say even less - I'd only go up to about 50" wide for diagonal piecing - those big triangles cut off feel wasteful to me, and I'd rather have long strips I can use for a future quilt.