The fomular if i am correct is 60% of the bet on the first selection then add your winning from that race plus the remaining 40% together to get a total amout then bet 60% of that on the next selection...gee i am confuseing myself will give you example

$100 bank
1st bet is $60 it wins pays $3 return $180 add to this the remaining $40 to give you $220
2nd bet $130 rounded down it wins pays $2 return $260 plus $90 left over from last bet gives you $350
3rd bet $210 it wins pays $4 return $840 plus $140 left over from last bet gives you $980
4th bet $590 rounded down now this bet looses but you are still left with $390 this the way to go...you are not resricted by venue work it out useing normal parlay rules it should be close