Trigger firing after half way point in race

I need help with getting a trigger to fire not too early in a race. I have worked out how to get it to fire after a certain time but with different length races it still fires too early too often. I have seen the "Acting towards the end of the race" and another post where you said you cant do the same in metres but i was wondering if there was a way to write in to allow 5 sec for every 100 metres of races in Oz. I know it would not be the same in all races but my aim is to get the trigger to fire inplay in the second half or there abouts of the race.

Hi
If you are estimating 5 secs per 100 metres
then 1 metre = 0.05 secs

conditions for betting trigger:
market's status is in-play
and selections trigger expression (now_time-event_off)/mf_second is greater than (market_length/2)*0.05  

Many thanks Mark, I'll give that a go.