Marking can be one of the toughest things that a professor has to do. I have some guidelines that I go by that have been suggested to me by other professors:
You cannot mark all levels of different subjects the same. In general I stick to some basic rules that help depending on the level I am teaching.
If it is a first year class, I generally allow about three to five errors in a written document before I start to deduct marks for grammar, spelling errors and punctuation. Especially at the beginning of the course when many of the students may have come out of high school, I try to make allowances that they are still learning how to write. I do not tend to completely re-write their text, unless there are some major problems. I try to keep in mind that not everyone writes the way I do and that I should let their individuality and creativity shine through their assignments. They are not in school to learn how to write like me, they are there to learn how to write their best.
If it is a second year class, I generally allow only three errors at a maximum before I start deducting marks.
If it is a third year class, I allow about two errors. For a fourth year class I allow one. If the class is a post-graduate certificate class, I expect flawless copy and will start deducting marks at the slightest error of spelling, punctuation and grammar.
I find this to be a fair way to do my marking. Other things I keep in mind when marking is to make sure I am really impressed with the work. If I am really impressed with what the student has done, generally they will receive A standing. If I think they could have put a lot more work into the story or written work, or audio assignment...they receive lower marks.
