I'd assume that Carmack was going by the same distance that the key was depressed in the reference frame for each test. The variance on that would be about one

I'd assume that Carmack was going by the same distance that the key was depressed in the reference frame for each test. The variance on that would be about one frame, would would add up to 4 ms for each test. However, since the input device did not change between the experimental equipment, it's a constant error that would be added into every test, making that a wash.