Timing

Now that I have spark and the slant 6 is running , I want to time it. I have adjusted the valves set the dwell but I am unable to get it timed to the exact spec. I can come close but not exact. If I remove the distributor and rotate it clockwise a tooth I still can not get it exact. If I remove the distributor and rotate counter clockwise same problem. I can get it within 10-15 degrees either BTDC or ATDC but not to 2.5 -5 BTDC as required. The timing mark is not wandering.
Thank you,
Otto