Using back price and stake to set offset liability bet

[vetinry]

Hello all
I've had a scan through the automation files but can't find anything to get me started. Also having a bit of brain freeze so thought I'd put a question on here and see if anyone can help.
I'm working on a strategy to trade potential steamers as follows

Place a back bet on a horse which I believe will shorten in odds first thing in the morning eg £10 at 10/1
Shortly before post time, place a lay bet removing 75% of the liability.

Back bet £10 at 11.0 - profit if wins £100
75% of £100 is £75 - place lay bet using liability of £75 if selection has shortened to 5/1 (6.0) this would be a lay bet of £15
If horse wins, profit is £100 - 75 = £25
If horse loses, profit is £15-10 = £5

If horse drifts in price rather than shortens then I also want to manage my exit position for eg 25-50% loss of initial stake

I'm not sure how to set the liability of the lay bet to be 75% of the initial back bet ie stake x odds - how do I get Guardian to store the information of what's been matched so that I can achieve this.

I hope this makes sense. If there is already a file that does something like this, I'm very happy to take a look at it and see if I can make it fit my idea

Thanks in advance

Steve