^Since I use several different mics, record many different types of music in many different venues, (plus my general technical failings) I'm unlikely to be able to do a decent job of the calculations you describe, but thanks for the suggestion. I will look into it. I'm mostly using the spdif input on the DR-680. The mics I'm most likely to use are km-140's, 414's, and AT 4051/53's.
After getting a little too hot using my AT 4051's on the inaugural flight, I've been setting it more conservatively, figuring that it's better to be too low than too high--especially in the 24-bit realm.
Thanks everyone for the feedback!