If you get a "Red" Flexi Trifecta Form then you just mark in your total spend. Then fill out a box trifecta with "x" amount of horses.
Melb cup is a bit different as pool is so large and no horse has very short odds.
Ok, that makes sense now. Think I'll definately have a go now... Anyway, good luck with your tipping, as well as anyone else having a punt...