Hi Mark.
I have the 550D - or that is what I used to take the image.
Yes, I sort of remember what you explained about ISO/shutter speed. But have forgotten the "extra" numbers.
So where I went unstuck is when I pushed the ISO to a high level, and got the shutter speed to 30 seconds, then wound the ISO back to a less noisy level, but then I didn't know the shutter speed time.
But thanks, now reading your reply, it is "double" if I read it correct.
So if I have ISO 400 and 30 seconds, and take it back to ISO 200, it becomes 60 seconds - right?