The stop block is what does all the “math” when spacing holes. It has two dowels that fit into the holes in the fence and “lock” it into place. For ½” spacing, just move the block to the next set of holes as you drill. But there’s a trick you can use to drill holes spaced ¼” apart. You can see in the Top View that the holes aren’t evenly spaced from the block’s edge. That’s done to allow for ¼” spacing. Here’s how it works: After drilling a hole, flip the block over and insert the dowels into the same holes. After drilling the second hole, just flip the block over, move it over to the next set of holes, and repeat the process. Once you do it a few times, it becomes second nature.