Sorry to come in late, but the way it normally works, all science GCSEs are made up of 400 UMS points.
Where EACH module is worth 100 UMS and each grade is worth the following:
100 - Full UMS
90 - A*
80 - A
70 - B
60 - C
50 - D
40 - E
So, in your situation, you got a C, for this sake, by last years boundary that is 67 UMS.
To get an A* overall, you need 90 * 4 = 360/400 UMS
So, with the C banked, from the other exams you will need 293 points overall, which is an average of (310/3): 97 points per exam:
i.e. You will need to get an A* in each other of the other modules..