If you don't like starting and stopping in the middle and dealing with loose threads, what if you went horizontally all the way across the quilt through the center of all those 6 in. blocks and also vertically through the center of all of them? This will end up with a + in the middle of each 6 in. block. (You would not start and stop at the block. Go from edge to edge of the quilt just like you did with the other quilting.)