Back bet at BSP specific rank
You won't know what rank they are until after the SP is done, so to bet on a selection by favouritism or specfic ranking, you'll need to place the bet once in-play - which could of course mean you don't always get matched
Assuming you wanted to place a SP back bet on the 2nd fav
The rule would be applied to the selection 'By Position (sorted in order of favouritism) 2
and armed to trigger at 00:00:00 till 00:00:05
You could use a relative odds condition to test there is 'X' ticks or odds between that and the fav and/or 3rd fav
https://www.betangel.com/user-guide/relative_odds.html
So if they are close in the betting come post time (ie, within 1 odds of each other) the rule won't trigger as there order may change when the SP is calculated, but providing there is a large enough gap ie, the 2nd fav is 3.8 and the fav is less than 2.8 and 3rd fav greater than 4.8 then the rule will triggers at it's 99.9% it will go off 2nd fav unless it does something stupid in the stalls or when walking to the tape