I wouldn't call myself an expert, misterg... It's more than a year ago that J introduced me to the exercise and we only brushed the subject when I was in London. So I could very well be wrong!
I only know this: the idea is to do 1 fret per finger, starting at the 7th fret** when you first do the exercise. Stay there until you can comfortably reach across 4 frets, so 1 finger in each fret. Then move 1 fret closer to the nut and repeat. The idea is to not move your thumb about too much, and you need to feel the stretch in your fingers, which is why you shouldn't do it for more than 5 minutes at a time, or you might hurt yourself. Only when you've reached the point where you can stretch across 4 frets starting from the 1st fret, are you supposed to try and reach across 5, starting again from the 7th fret**.
** Or whichever fret suits you best; you need to feel it, but it shouldn't hurt.