Hi
formula for percentage price change is (current price - original price)/current price *100
so you would have a second laying trigger to lay at lay_price in play with these or similar conditions:
selections lay matched is greater than 0
and selections trigger expression (lay_price - bm_layp)/lay_price *100 is between -25 and -20
if you wanted the original trigger to be modified for you it is best to put in a
trigger request