11.25″ from the bottom. attach a 35″ 1×4 between them with glue and 2″ screws, with the top edge 12″ from the bottom of the legs, and 3/8″ in from the edge. note: having properly squared ends on the 1×4 will help keep the whole thing square (don’t trust the factory cut to be perfectly square).
attach the 8.5″ and 10.5″ filler pieces to the back legs, using glue and nails. the gap between them should just fit the width of the 1×3 rail. attach the rail and the lower back support using glue and 2 screws on each end, 3/4″ from the corners.
attach the short pieces of 1×2 to back 