Sounds more like a personal vindetta rather than a critical grading error. It sounds like you have the courage to stand up for your grades and to argue them for what theyre worth (in your case 47 points).. but dont get too overzealous and make a fool of yourself. Assume you have made a mistake until you prove them wrong that way you always appear humble and gracious (even if youre steamed). Surely there is reasoning behind the grade that you were given... unless you are opting not to bring it up.