I have this meter as well.
I'm wondering if you are aware of the battery issue with these?
The meter is designed to use the old (no longer generally available) 1.35 V mercury cell, apparently, replacement with a regular alkaline battery will screw up calibration.Just changing the offset or "null" will not fix this if the linearity compensation of the unit is designed around the load of the original cell.
There are a bunch of threads around on this on various forums, and there a few options are available (adapter, Wien cell, etc.).
If you are aware of the battery issue and think your calibration problem is unrelated please disregard-