Personal experience is the best teacher. Mark's recommendation to test your usual 100 speed film using 50,100 and 200 sounds like a great idea. Rather than use three separate rolls, why not just meter for 100, shoot, then expose 1 stop more then 1 stop less on the same 36 exp roll. I think it will be well worth your time. You could read photo magazines for years and not learn as much as from this one simple experiment. I hope you will take his advise . If you do please report the results.