Guardian - Placing bets at certain win probability

[Airliner]

Hi, I'm trying to place a back/lay bet in guardian, at a certain level above or below based on chance of winning, relative to where my last back or lay bet has been matched.

Example;

I place a back bet at the best market price, lets say it's matched at 4.0 (selection has a 25% chance of winning), store the value of 4.0, and then I would like another rule to place a back/lay bet at a certain win probability level above or below.

If I wanted the 2nd bet to be placed at a level above (minus -5% chance of winning), the rule fires in the bet at 5.0 (20%)
If I wanted the 2nd bet to be placed at a level below (plus 5% chance of winning), the rule would fire a bet at 3.33 (30%)

Hope the above makes sense, wondering if this can also be done based on SP, rather than the price of the last matched bet?

Thanks