I'm not the expert here. I'm just saying that it ought to be possible.
So you believe that a Spectrophotometer, and a car, have the same degree of accuracy over the same span (in your example, its the same from 20-80,mph?).
It's perfectly possible to say over what range of speed a speedometer will work to a specified accuracy. Why can't a similarly appropriate set of variability over a known range be specified for a spectro ?
You can insert any other form of measuring device and variable, It's what the science of measurement is based on.
A procedure that some recommend once a year....
Yes, it's not ideal, but I'm pretty certain that not many owners make the investment in annual re-calibration of their i1. The clue was that when I asked X-Rite they didn't immediately know how much it would cost or how long it would take, rather a give away that they're not dealing with that service very frequently.
It would be fascinating to know how the ones they do re-calibrate have drifted over time and if they still meet the criteria you've quoted, I wonder if they check ?
You got that data how?
Consistency of readings can be done by repeatedly measuring the same targets and comparing the results. I've done it a few times when I've been interested enough and had some spare time.
i1 to i1 is harder as I've only access to two instruments, but again doing a similar test to above you can start to get a rough idea of what's going on.
I'm not defending it as definitive data, but it's more than I've read of anyone else doing.