My primary desire is to blur water for waterfall shots, and some lake/ocean shots. I have been able to do a little with my polarizer but it is limited. What do you all recommend as a good value to start with? I'm guessing that it may be best to err towards the high side but I don't know. Also, do you usually guess what might be the largest lens you would ever use with it, buy that size, and use step rings, or just end up buying two or three filters? Thanks!