Ok here, this is to give you an idea. Hope they came out clear....... Anyways, Your looking for a straight wheel and sprockets and chain to line up, with correct slack in the chain. Your using the straight edge to site this. When I do it, I don't care were my snail notches land.....so long as it's straight. And i don't rely on marking the the snails.... because the next time you do it, it may be different. You can go ahead after doing this, and check your front to rear wheel alignment if you want. It should be close. If it's not ? Something is up........ Sprockets should align as well as front and rear wheels.