Well - let me try a couple pictures of my own to make sure I don't get too carried away here.
This shows the flush edge I currently have on the first wheel well I turned down with a sanding drum. It's a little fuller than I thought - just slightly less than 1/4 inch. Bob's 3/8 inch has me a little worried as I'm already a little below that on this face although when I roll the edge in the thickness normal to the fender will probably still be 3/8 inch or so at least.
This shows the lip return - except I hit it with the sander and probably have removed 1/8 to 3/16 inch thickness from it already. I'm trying to get an idea of how much I need to take this down in thickness to lay the rolled edge back on the inside.
Do I look to be on the right track or have I gone a little too far? Pictures of finished cars like CobraJeff's give an impression of a fairly knife edge fender lip but that just may be the photos.
Thanks for the responses.