partypooper, if you can give a more precise example then I could do something up in excel i'm sure of it.

i.e. give an example of say a 3 race sequence where the first 2 races both horses lost and i'll do up a spreadsheet that will calculate what your after.

Is the following essentially what your after? also what determines how much is placed on your #1 pick?

Race 1:
$10 to be placed on Horse 1 @ $6.50
Horse 2 needs to recover own stake and $10 @ odds of $4.10
$3.23 placed on Horse 2

If both above lose,

Race 2:
$12 to be placed on Horse 1 @ $5.80
Horse 2 needs to recover own stake, $12 from Horse 1 and $13.23 from previous race @ odds of $4.30
$7.65 placed on Horse 2

If both above lose,

Race 3:
$11 to be placed on Horse 1 @ $5.20
Horse 2 needs to recover own stake, $11 from Horse 1, $13.23 from Race 1, $19.65 from Race 2 @ odds of $5.60
$9.54 placed on Horse 2

Assume that one of the horses wins Race 3, your either break even after 3 races or in profit, Assuming you take into account commission if on BF as well.

Is that example essentially what your after?

EDIT: All odds are just an example, essentially the odds don't matter because the equation takes them into account, all that really matters is your staking on Horse 1 and the Odds of each horse in each race.