I find flats invaluable. I do have some vignetting and trying to keep the sensor, filters and other glass surfaces dust free is very difficult.
I have a light box by Exfso that came with my scope that has a potentiometer on the side to alter the brightness(have another Exfso light box too, good stuff!), 3/4 for LRGB and flat out for narrow band. I generally aim for about 30,000 which gives a nice flat and cleans my images right up, simplifying my processing.
Generally I let the software take care of the maths, although I am sure it does all the bits as I see it in the processing console as its working(PI is my processing software).
I think the real trick is to work out what the best flat for your camera is using a very even light source.
Interesting read thanks Greg.
