Just another angle that some may like to try. This can be done race to race or meeting to meeting. take first 5 in betting, bet $2 all up the two favs, + $1 all up the other 4 with the fav in the 2nd race. Now say the fav won, check what your expected return is should the 2nd one win eg. First fav won 6-4, next one is showing $4 so return is potentially $20 with $6 outlayed. so now decide taking the best possible prices from different bookies what to cover from 2nd to 5th fav. in 2nd race.

Now say the fav in the first race lost but you hit the winner in 5 , repeat, calculate the potential returns should the fav win in the 2nd race, and cover to profit or break even the other 4.

Now you will hit most doubles, but you have the added benefit of knowing what the potential return is B4 you make the bets on the 2nd race, and of being able to make your bets where you like or tote.

PS I've used $1 unit here for demonstration purposes only.