Since these questions are self-grading the way you actually do it is up to you. It would be interesting to hear how other people do it. I, for one, am grading myself rather leniently the first time. When I redo it after a while, I'm much stricter. 

For example, I don't take off points for simple spelling errors. And avoidable errors that I really do know only detract one point. I kind of go by how well I feel I really knew the answer and not so much by a strict, objective scale.