Just so everyone knows, FAR 43 appendix E is where the standards are for FAR 91.411 testing of altimeters and reporting.
At these altitudes the allowable error for the altimeter indication is + - 20 ft. This is whether it is a pneumatic gauge or air data computer derived.
The standard for altitude reporting in any format, Mode C or Mode S is + - 125 ft. from the altimeter indication at any altitude.
Just offered up for reference.