Servant to close trade and leave scratch on other runners

[Dadders]

I would like to create a Servant for in-running to use on L2B trades.

If the trade starts going against me I would like to set a Servant that creates a red only on the horse I am trading and leaves a scratch, £0.00, on all the other horses in the race.
I would like it to work like a Global offset bet where it triggers at say 20% and actually places a position at 25%.
Is the best way to do this by using the rule 'close the trade,' do I need to add anything else to create the scratch on all the other runners ?

Thanks for any help and advice.