Dutching - Building bBook up to 80%

[Sjpiercy]

Hi all,
I am currently setting up some automation that will build a book up to 80% coverage and place back bets providing the result would be profitable.

Thus far I have set the file up so it limits races to those that contain between 5 and 10 runners.
I have also added rules that will make sure at least half the field is selected, i.e if 10 runners then back selections 1-5. If 8 runners back selections 1-4.

Problem I am having is setting it to place back bets on selections until around 80% book is covered and overall profit condition is positive.

Can you advise???

Regards

Steve