What is the maximum allowed difference between two altimeters for flying in RVSM airspace?
On the ground during the preflight check, the altimeters must be within 75 feet of each other.

In-flight the altimeters must be within 200 feet of each other.

I have been flying the B777 for about 8 years and I have never seen the altimeters disagree by more than about 10 feet, either on the ground or in flight.