It would work that way for win bets but place bets are different as there are 3 payout options and there would be know way of know how much was bet on your selections.

If you used to above example you could state that this would be the maximum amount the price would drop because you are using the whole pool amount not the pool amount on that horse.

I also think you need to remove 18% from the pool before calculations.

__________________
One Drive

"If the corporates are treating you poorly , just go elsewhere."
"If they need you , they will soon find out."
"If you need them , you will soon find out." --moeee
_______________________________________________