True center-shot is an oernery donkey to try and pin the tail on. The way the cables come off the cams and how they react through the draw cycle may cause center shot to change as the bow is loaded. It may be that your bow is at center shot for the way it cycles, but 1/4" seems like a bit much to be out with the bow at rest. I'd say if the groups are tight out to 30, and broadheads are falling in the same group then you're good.