For under twenty dollars, you should purchase an electronic stopwatch or timer. This little gadget will make sure that you make the best use of the time allotted in slow fire. Without it, you will probably shoot too quickly, wasting the opportunity to wait for the wind to stop blowing or for your arm strength to be restored after a shot. The inverse is a bigger problem, having taken too much time and still having two rounds left unfired at the end of the time. It allows you to determine whether those borderline shots are actually touching the scoring ring.
Some overlays have a small arc of a few inches of the four innermost rings of the target.
They are cheap. Get a set. Also use a notepad for recording your scores. This can be an invaluable training aid. As you keep track of your scores, you should also make notes during the course of the competition that may be referred to later on.