Hi Rick, check out this link. It explains center shot pretty well. I wouldn't suggest their way of finding center shot however. One thing that hasto always be assumed is that the individual doing the tune is very consistant,and that there is no torquing issues at full draw. Ofcourse that is never 100% with a person.
http://www.skookumarchers.com/Archery%20Library/Compound%20Bows.htm
Anyway, how did I used to get centershot without calling the manufacturer or using one of the gizmos out today?
I used toplace my compound bow onthe rollers of mypress and put a levels on the back of each limb at the pockets. Then I'd take a very long arrow (usually the samediameter that I planed on using) and put a 220 grain field tip on it, place it on the string and let it hang freely while checking the levels (limbs parallel to the floor).Then I'd match the left right adjustment of my rest to match perfectly. This would get me extremely close assuming there is not a limb twist problem.