Not sure how others do it? I will hold the bow in one hand extended out and line up the string with the center of the top and bottom portion of the riser fasing you. With an arrow nocked the shaft should be in line with the string. If it is not I adjust the rest slightly.

How do your arrows group at 20 and 30 yards? I have seen some guys put a vertical piece of 1" tape on a backstop and shoot it from 10, 20, 30 or 20, 30, 40 yards. Don' t concentrate so much on hitting a certain spot, just aim at the tape and mark which arrow is which. If all 3 arrows are on or close to the tape you should be okay, if they start to drift one way or another more the farther back you get you will have to adjust the rest.

Hope this helps, and Good Luck.