I found this little uk system during an internet search. One of 3 that were about the betting 5 to 6 horses in a race.

The basics are if you bet level stakes you would lose in the long run. So you have to bet to cover your stakes for each race until you win and make a profit then you start again.

So, 6 horses in race 2
Stake $12.00 @$2.00 each.

5 horses in race 3
Stake $24.00 @$5.00 each rounded up.

Somehow I'm not sure about this. Any ideas on how to cover your stakes from the previous race for the up coming race.