Altimeter limits
On most certifed aircraft, the altimeter tolerence at ground level is 2mb - approx 60 feet, so over 100ft is not in calibration. The mode C tolerence is +/- 125ft, so with a 100ft readout (not Mode S with digital encoder which will have a 25ft readout), the altitude should round to the nearest 100ft, thereby a 125ft error will show as 100ft out, whereas 175ft error will show as 200 ft.