Hi nutloose,
In my experience, the acceptable difference between two altimeters is not the same depending the altitude.
On ground, the maximum permitted is 1Hpa (20 feet) but that difference can be higher in altitude. At FL 300, 100 feet of under reading is nothing, but during short approach in CATII, you can be in a deep trouble.
In all companies I have worked for, 100 feet difference ON GROUND is a NO GO situation.