Test tolerances here in NZ are +- 20ft at 0 feet, +- 75ft at 40% of max altitude. That said I'm sure one of the local pilots had a look the other day and said the operational limits were something like +40 to -30 feet (or vice-versa). I recall the discussion as we all had a stab at what we thought was correct and he went away to check on it and came back with this slightly odd limit.
Interestingly the reason for the discussion was that one of the light a/c here was reading 100ft out (don't recall which way) on the ground given the local QNH. There were a few theories abounding (the pressure measurements are made a few miles from this airfield) but it turned out to be consistently awry so looked as if the instrument was out & we had it adjusted.
I think many of our rules are based on the UK ones so I'd think your requirements would be similar - would be interested if you find out.