The half square triangle formula won't work because the finished unit is a rectangle. EQ says to cut strips for those patches at 17-1/8" x 10-11/16", then slice in half along the diagonal to create the two right triangles. If you want to construct the block by sewing two rectangles together, I would bump the dimensions up to 17-1/2" x 11" and cut two rectangles that size out of scrap, sew them together, cut down the middle, press and then see how big the unit is. You may need to sliver trim to achieve the 10-1/2" x 16-1/2" unfinished size, or increase the dimensions of your rectangles until you get there.