[rushyrulz]

1/n(c1 +/- c2 +/- c3....ck)

Where n is the number of players, c1, c2, c3, etc are the points changes:
9 or more: 0
8: 20
7: 30
6: 40
5: 50
4: 60
3: 70
2: 80
1: 90
0: 100
-1: 110
-2: 120
-3: 130
-4: 140
-5: 150
-6: 160
-7: 170
-8: 180
-9 or less: 0