What type of aircraft is it?
Most flight manuals on PA38/PA28 type aircraft list the max pressure error correction as plus or minus 50 ft.
That said there are other factors. The Altimeter check should always be done on QNH whilst parked on the apron. Apron elevations are listed in the UK AIP for major UK aerodromes. Without listing all the arguments if you are using QFE there may be other "errors" as you may or may not be using a threshold QFE or aerodrome QFE and not at the "reference" point.
Also when the pressure is measured it is always corrected down to the nearest millibar. E.g measured pressure 1000.9 mb, pilot is passed 1000 mb - in extremis this means a .9 mb error which equals 25 ft (1 mb = 27.3 ft). Hence the quoted of limits of -75/+50 ft (the extra 25 ft on the negative side to cater for the possible .9 mb difference).
If you have more than one altimeter then EACH altimeter should be within the tolerance so you might have one altimeter on the minus 75 ft limit and the other on the plus 50 ft limit, hence a difference of 125 ft!
Hope this helps!