formula/ trigger advice - place a lay bet before my unmatched back bet is filled

Hi,

My trading trigger is partly automated,  however to completely automate it i need to place a lay bet before my unmatched back bet is matched.

example: I  have an unmatched  back bet placed on the first fav at odds of 3.20 when lay price is one tick away from my unmatched back bet i want to  place a lay bet. So the lay price is at 3.15 i want to place a lay bet as its 1 tick away from my unmatched  back  bet of 3.20.

How do i write a trigger to do this?

Thanks

Hi

lay, price: lay_price
selections lay price is equal to r_ticks(bu_backp, -1)