Hey Rob,
Glad you found the right sweet spot for your gear. Remember, you want the highest level you can get going into the Fostex without clipping. Also, according to the Fostex website (and not uncommon for other units) the clip lights can start to flash on occasion without any clipping actually happening. You can hear clipping because it sounds like crunching, crackiling garbage -- not like a little distortion, ala analogue tape.
So, you really want your input level on the VF80 to be consistently up near and touching the very top line on the meter. Same thing when mixing down and mastering. Otherwise on your final CD, if you have to add a lot of volume at playback, you'll be hearing significant noise. This will be very apparent on solo vocals or classical guitar... Also, be aware that on the Mastering sequence, the default for the VF80 is to have the Compression ON. If you use compression, then during the quiet passages, it may tend to boost the silence -- thus raising up backround hiss. Try not using the compression when you master -- particularly with the classical guitar. Another trick... When your track is mixed down to 7/8, then erase the section between 0 and the actual start of the music. Use the WAVE Scrub function to determine this. This way, any ambient noise that was present before the song actually starts will be totally gone.
The VF80 is capable of VERY, VERY quiet recordings, but it is also very capable of reproducing any ambient or extraneous noise that is present. Ah yes! Digital accuracy!