So far, I've identified these flaws with my Defensive Winning Percentage:
1) The statistics will, by their nature, include plays made on balls out of a fielder's zone. This is not necessarily a good thing.
2) The Positional Adjustment, being based on traditional Zone Rating, doesn't accommodate flaw (1). Thus, one ends up with estimated Zone Ratings in excess of 1, which is a logical nonsense. What I should do is adjust the positional factor by the distribution of plays on each team, but that opens up a nightmare world of a lot of work. Another option would be to cap the estimated Zone Rating. Or you could leave it, and bear it in mind when looking at some of the data.
3) There's no adjustment for groundball tendencies of pitchers, nor handedness of batters. This is quite conscious, in part because I don't see the point of doing this for a metric trying to measure what has happened, as opposed to talent level. In other words, I think a player whose pitching staff is pitching to his strengths, deserves credit. I may yet incorporate these factors, but at this stage I get the impression that on the whole the measurements even out.
4) It really is only what I'd call reasonably reliable as far back as detailed statistics from Retrosheet go, including the all-important doubles and triples given up by a team's pitchers. Without those, I'm more likely to believe conventional wisdom where my system challenges it.
I intend to stick with this unsatisfactory metric for the time being. I think it does a good job at giving us an idea of the relative ability of a team's fielders at a given position for an era before modern play-by-play metrics. That's all I ask of it, and that's all I ask you to ask of it.