I think we need a totally different mechanism for counting characters, so that we don't have a constant tension between "x should logically implicate y" and "but then the char tags will be misleading". Both are usually valid points, so the solution would need to be in functionality.
Brute force solution: add an additional field independent of the chartags: metatag where we can manually select a number of characters. We would use human judgment to count rather than trying to shoehorn conflicting desires into our tagging policy.
It could work in addition to the chartags: metatag, rather than in place of it. "numchars:" perhaps. It could be populated during tagging by typing in numchars:3 or with another text box when editing.
Obviously the challenge to adding anything like this so late in the game is: IMPOSSIBLY MASSIVE BACKLOG. But that shouldn't necessarily stop us from something that would be useful going forward. At the very least it's an addition of functionality, not a removal or replacement or alteration of existing functionality, so it doesn't harm anything if it doesn't take off or misses legacy content.
Or we could just say this isn't necessary and live with chartags: being imperfect, which works just fine for me as well. But a lot of people don't seem to be find with that, so here's an idea. Not an elegant idea, but likely an easily-implemented and simplistic to understand one.
Updated by Katajanmarja