ORIGINAL: Straightarrow
The size of your groups at one distance, are irrelavent. Try as suggested, the walk-back tuning method. That is how you determine if centershot is correct.
Straightarrow is correct. I had the same problem. Did the walk-back thing, established center shoot and was able to adjust my sight back towards the riser. It's been stated here many times that paper tuning should be the starting point.