A 16.5 panel will finish to 16 inches on a side. that means that the finished size of whatever goes next to it also has to be 16 inches. So four 4 inch blocks (raw size 4.5)works. In order to put three blocks on a side they have to finish at 5.33 inches, so add half inch to that finished size for seam allowances and they have to be 5.33 + .5 or 5.83 raw. I can't cut that measurement, can you? Your plan for 5.75 inch blocks will be slightly short. Any multiple of 4 will work, like two 8 inch blocks, but then you are going to try to divide the 8 inch measurement by 3 if you really plan to use D9P blocks, and again, you will have a hard time getting it to work. You could put a small, plain border around the panel to get it to a number easily divisible by 3, like a 1 inch border (1.5 raw) which increases the panel to 18 inches and now you can work with the D9P design. Clear?