Dan,
Thanks for that info. That answered a large part of what I was asking (as far as more magnification for a given extension length and same flange to subject distance at a given magnification). I've been doing MF macro work with extension tubes since the 80's, but I haven't done anything much more than 1:1, and I haven't reverse mounted any lenses.
I understand how reverse mounting a wide angle lens gives more magnification for a given extension. what I was wondering is if there is a general preference between more extension with a standard lens, or less extension with a wide angle lens, for a given magnification level (say 4-6:1), assuming I have a bellows so that I can get that much magnification with a standard lens (and of course, the lens would be reverse mounted either way).
On a side note, are there any other good online places to find used MF gear besides ebay and KEH (preferably ones that take paypal)? I've been looking for a reverse mount adapter for a while and haven't found one yet. I found a way to improvise until I find one if I have to. I have a collapsible rubber lens shade that fits snuggly over a macro tube (I actually have to stretch it a bit to get it on the tube). In a pinch, I could use it as a reverse mount adapter.