The thing with Further Maths modules is that people only take them if they're good at maths. This means that the UMS curve is always going to harsh. Exactly why your raw 89% in FP3 turned into 79% UMS. One raw mark will probably bring it up about 3% and so on. Maths isn't very good with remarks because you're either right or wrong. Who knows, you might get an extra mark or two but unless they missed a page, unfortunately I strongly doubt it will go up too much. You need 12 UMS over FP2, FP3 and M2 for it to go up to an A*. I'd say the most you're likely to get is probably 3/4.