G'day Swooper,

Here is a formula.


X * [( X * Y)- 1] * [ Z + 1]
........................................ -1
X + 1

Here is an example

Horse X=2/1
Horse Y=3/1
Horse=4/1

2 *[(2*3)-1)]*[4+1]
................................ -1
[4+1]

=> 2*5*5
.......... -1
3

=> 50
...... -1
3

=> 16.67-1

=> 15.67

=> 16/1 rounded

This formula is very accurate,BUT can be distorted if the net probabilities of any combination is in excess of 100%

Hope this helps.