So, if I only develop totally unexposed film, how much Xtol should I replenish for each roll? Also, what would it be if I only took pictures of 18% grey cards?
In the first case, it would depend on the fog level of the film, but you'd have to develop a dozen rolls or more to see the change in activity with standard 70ml/roll replenishment rate (and you'd have to develop something with images, or at least a step wedge on it to
see that change in activity). For a gray card, if the frame is filled with the 18% gray, you should get the same long-term response to replenishment you would with "average" contrast scenes, correctly metered. That is, 18% gray should give the same overall amount of developed silver as a normal scene, on average.
Of course, the other result is that your wallet will slowly empty (or not so slowly, depending how much of this you do) without having any prints or scans to enjoy.
I recommend actually making pictures and following Kodak's instructions on replenishment. You'll have much more to show afterward...