Benny, just my 2 cents worth mate, I've been "investing" for MANY years but have only made a profit in the last couple.
My experience of this type of betting is a LOT of effort for very little return IF any. (I'm bracing for a barrage of abuse there,) but I'm only stating my humble opinion that you should not look further than 3 selections per race, and also explore the possibility that you back them at level stakes, within some other staking plan, in other words the 3x1 bets are treated as 1 bet, so say you have 5-2, 5-1, 7-1, bets are at levels so say the 5-1 shot got up then your bets results in a return of $6 for a stake of $3 in other words 1-1.

Over MANY bets I found that this worked out about the same as dutching WITHOUT a hell of a lot of work!!