The pre-flash mode for the C645 is performed in spot mode. So Marc's suggestion makes perfect sense.
Normally, I use mono lights for all my flash works, and meter my sense with my Gossen StarLite meter. Hence, my experience with TTL is minimal.
Last month, I shot a wedding with the TLA360 and my N1 + N24/85. I used the flash for every shot in TTL, including day light indoor, sunny outdoor, and night time party. All 500+ pics came out perfect as I followed Marc's suggestion from the other posting.
Before that, I did a party with my N1 in fully TTL mode, most of my flash pics were over-exposed in the subject, especially the wedding gown. And the blackground was under-exposed.
What Marc suggested was, use manual mode with low shutter speed, set the flash to TTL and let it does the job (compensate). The idea is to allows more light comes through for the background.
With that pricinple, I set the camera in Av or Tv auto mode, set the apperture wide enough, and/or set the shutter speed very low, then set the flash to TTL. The result will come out great.
Colin, You don't really have to meter every shot and set to AEL. The idea is use the TLA as, kind of, a fill flash, meter the sense as if no flash being used, either auto or manual. Even the meter show one or two stop under, or over, it will come out okay. You have to try it out. It is the pricinple of lighting, has nothing to do with Nikon or Cannon. Although I agree the N or C has better (newer) eTTL.