Re: How far "should" I cast?
I think that the term "accurate" increases in scale in direct relation to the distance.
In other words, if you are casting 20 to 30', you probably want to be able to put the fly within 6 or so inches of your intended target.
As that distance increases, so does your range of acceptable accuracy. Of course, it would be fantastic to be able to put a fly within 6" of your target from 90' out but I am guessing that there are only a handful of people that could do that consistently. I know that I can't.
Instead, I try to be withing a foot or two when I am casting 75+ feet. My targets are different at that range though. I am not trying to put the fly right in front of a fishes face at that point. Instead, I am trying to cast to an area that looks fishy or even a shadow. I am trying to get a fly to land in a small cove on an otherwise flat bank. I am trying to hit a point of a lily pad base.