How far should one be interpretively charitable? I don't think there are rules; it is mostly a matter of balancing one's confidence in oneself and one's confidence in the author. If you are reading something that is very hard to understand, then generally you should read it in such a way that it is at least coherent and intelligible. But what if the author really does not make any sense? If you have enough confidence in yourself, you will find out fairly soon; if you have more confidence in the author, you will be wasting more of your time.
Just this morning I was commenting on a paper, and the comments will have to be sent to the author. When I first read the paper, I found it almost impossible to understand. I was so frustrated. Should I simply write "This paper is unintelligible; I have no idea what the author is trying to say"? No, such comments would be unacceptable. Besides, the author is a well-known philosopher. What he wrote must make sense! I forced myself to reread the paper a couple of times. Finally the paper became (more) intelligible, and I managed to write some comments that I think were sensible and reasonable.
I couldn't help asking myself: Would my comments have been very different if he was not a well-known philosopher?