[acropolite (Phil)]

I can't see why you would need any more than 1 dark frame for a given exposure. Surely you're subtracting noise, which would be relatively constant for a given exposure time. I know my 20D has an inbuilt dark frame subtraction when you turn on long time exposure noise reduction and it takes a dark frame of an equivalent time to the exposure time used. I can see it may be useful to build a library of dark frames for varying exposures and ISO's though, that way you could save taking a dark frame every time, but of course temperature would also need to be factored in.