Originally Posted by UselessBettor

Exactly as Shaun said. Lets use Doomben:

I want $50 profit. So I am laying $50 + cost of Quaddie in first race which is Race 5. So we lay 3 and 14 at Doomben.

Number 3 = $8,60 on betfair.
Number 14 = $3.45 on betfair.

Lay 3 for ($50+$128)/(1/8.6/(1/8.6+1/3.45)) = Lots of maths but basically this is saying find the percentage I need to lay on number 3 out of the $178 liability = Laying 29% of the amount on Number 3 = 0.28*178 = $49.84 (Round it to $50).

Lay number 14 the same way but 71% = 0.71 *178 = $126.

Number 3 one this race. So overall our loss on the race was 8.6*50 = $430-(50+126) = $254. Add this to our outlay already of $128 and we are down $382 after the first race.

I'll post Race 2 after its run and I've checked the odds on betfair.