I've struggled with the same problem. If I don't know the material 100% I panic and usually end up with something under 50%... even if i know the material like 90%... its weird. If I end up with a bad feeling after the midterm, thats when I tell the professor whats up. Most of them ( the ones that aren't total dicks) pretty much drill me on the material, figure out that i understand most of it, and take that slightly into consideration with the final grade... I think the only thing you can do at this point is tell him your situation, tell him that you would be willing to talk to him about the material, if he would like to test your knowledge, and hope for the best. Good luck!