It's all proportions, but it's hard.

Despite significant help from this board, I've pretty much given up.

Bill Heckel, a fly-tier here in the Chicago area, came up with the following pattern (mine are a little bit different from his).
[url=http://www.danica.com/flytier/sschwartz/half_spent_bwo.htm:33853]http://www.danica.com/flytier/sschwartz/half_spent_bwo.htm[/url:33853]

The biot underneath the hook splits the hackle in half. Due to the natural curve of the hackle, you might have to do a little preening. I also cut away any stray fibers, but the amount I cut is a lot less than would be cut creating a modern thorax tie as suggested above.

Vince Marinaro suggested something similar in "The Ring of the Rise." Datus Proper's tounge in cheek-named "Perfect Dun" used dubbing figure-eighted under the hook to separate the hackle - creating the same effect.

For a really cool fly, check out Roy Christie's Easy Peasy USD Mayfly which was a FOTW in the fourth quarter of last year.

Despite supposedly "sitting lower" in the water, I believe these flies have good floatability because of all the hackle sitting on top of the meniscus.

-Steven