For those who are still adding kanji, there is a very obvious measure of progress: the number of kanji in play, and/or the number of kanji in the low-numbered boxes that are displayed individually on the review page.

For those who have hit the end of the list, the number of kanji no longer changes.  I've been at 3,007 for a year or so, but I'm still reviewing and trying to work on cards to keep them from slipping out of my memory.  With fairly few kanji outside of the rightmost column ("box 4+") on the review page, it's hard to see progress.  I don't really know if I'm on the whole slowly moving kanji into higher boxes beyond #4, or just treading water, or even losing ground.

I was thinking that a progress score could be created, something like "1,000 times the average box-number that one's cards are in, rounded to the nearest integer."  That shouldn't consume too much database time -- I'd expect it could likely be picked up in the query that produces the review page.  The use of an average yields comparable scores for those who stop at RTK1 and those who continue beyond it.  The calculation should result in a four-digit number for most site users -- comparable to the number of digits in the typical "number of cards" display.  A successful review that moved just a handful of cards up one box would increase the number, even for someone with all 3,007 cards.

There seem to be a fair number of people who've hit the 3,007 limit but are still regular visitors to the site.  I wonder if they, too, would be interested in some sort of overall progress score, or whether I'm alone in this.