Thanks for the ideas, I think you are correct. I did some more detailed calculations and found very little difference in average file size between the two cards (41.0845mb vs 41.34851). Using the space left on the SD card would imply 742 shots vs. 738 on the CF card. The shots remaining number is 223 on the SD card with 329 taken, so implied 552 total. This is an improvement over the approx 478 estimated at the time the camera "overflowed" to the SD card, but still very inaccurate. Presumably as more shots are taken the accuracy of the "shots remaining" estimate will improve.