Hi,
You need to understand how the public will be betting to make good profits in the tri's,that is at what odds do they switch from doing a Standout with the Fav. to Boxing.

You'll find that the races at The Meadows an Sandown Park can be quite even an there are only a few Overlay combinations to be found in each race, cause of this effect of Boxing at longer Fav odds.

You should look at the Harville formula, with some slight adjustments, and use that as the basis to constructing your exotics, that combined with a Modified arithmatic staking method will ensure you will find it quite beneifical.