? can I take the folder # x 100 for a rough number of actuations on the 20d

Yes, but of course that number resets each time you reformat the card.

The actuation count variable shows you the concatenation of the folder number and the frame number. So the first three digits are the folder number, and the last three digits are the frame number.

The frame number resets to zero once it passes 9999. So if you have several sets of images that the frame number has wrapped around then you can multiply that number by 10000 and get a good approximation of the number of shutter actuations on your camera.

HTH,

-Kirk