Sometimes if you have a single rail site, where all the pins are touching each other you might have to set the twenty yd and keep walking back until the shot becomes to low, now try your 30, if it is to low also you can now move your whole site bar up until the twenty hits center then take a shot at 30yds and keep it up until both the 20 and 30 yd pins hit center. Maybe I confused myself and you also, what you are trying to do is set your pins that all are hitting the center.