My procedure is to adjust it down as low as it will go without contacting either the shell plate or a shell locating strap. Keep in mind that the lowest travel of the tool head occurs slightly before the handle reaches the end of its travel.
I check with a piece of paper to make certain that the sizing die never contacts a shell locating strap. The shell locating straps are hardend, and if you flex one enough times by pressing on it with a sizing die, it will break.