The key is converting Kg distance from the winner into a decimal.

Winner on ratings = 1.0
.5Kg from winner = .90
1.0 Kg from winner = .80
1.5 = .70
2.0 = .60
2.5 = .50
3.0 = .40
3.5 = .33
4.0 = .25
4.5 = .20

So...if you have 3 horses within 5kg of the winner and they are rated 60, 58 and 56 you add up 1.0+.60+.25 = 1.85. The top horse is thus 1.0/1.85 or 54%, the next horse is .60/1.85 or 32% and the last horse .4/1.85 or 21%

Does this make sense?