Okay, well I've gone from having the top rated runners a bit too short to having them a bit too long, so I've made another slight adjustment to the table and I think I've probably got it just about right now.

Top rated horse = 100 points
0.5 kgs behind = 87 points
1.0 kgs behind = 75 points
1.5 kgs behind = 65 points
2.0 kgs behind = 57 points
2.5 kgs behind = 50 points
3.0 kgs behind = 45 points
3.5 kgs behind = 40 points
4.0 kgs behind = 36 points
4.5 kgs behind = 32 points
5.0 kgs behind = 28 points
5.5 kgs behind = 24 points
6.0 kgs behind = 20 points
6.5 kgs behind = 17 points
7.0 kgs behind = 14 points
7.5 kgs behind = 12 points
8.0 kgs behind = 10 points
8.5 kgs behind = 9 points
9.0 kgs behind = 8 points
9.5 kgs behind = 7 points
10+ kgs behind = 6 points
11+ kgs behind = 5 points
12+ kgs behind = 4 points
13+ kgs behind = 3 points
15+ kgs behind = 2 points
20+ kgs behind = 1 point