laying condition

In a continuous laying trigger on the favourite what condition must be added to prevent the trigger from laying at prices below the initial lay price? If the opening lay bet is for 1.35 the next lay must be 1.35 or above. I've unsucessfully tried using  remembering the fav starting price.
Regards,   

hi
and fitting selections lay price is equal or greater than bm_layp.
this will place the lay bet if the lay price is equal or more than the last matched lay price on that selection
BUT sometimes you will get a lay bet matched below your setting because betfair will match your bet at equal or less than when lay betting.
mcbee