Thanks, I think I get it. The official literature gives the impression that the goal is a perfectly balanced exposure across the negative, but you're saying that the centre filter is more of a subtle cheat than a precise correction. And I guess that it makes more sense, given that the amount of compensation doesn't change, that the filter would be balanced so that its effect would appear across all apertures—but at such a level that the difference between one extreme and the other would be barely noticeable.
Or possibly I've been at work for too long and I should go home and stop rambling!
Anyway, thanks for the reply.