Or you could try something like this.

Saturday races, lay every runner that has won less than 1 in 6, provided they have had at least 10 starts, and horses that are still maidens after 10 starts or more.

Last Saturday 3 winners from 77 runners. They average around 1 in 16.
(previous Saturday saw Satin Robes as a lay)

Weekdays, lay runners with a strike rate of 1 in 10 or worse. Same rules as above apply for maidens.

Yesterday 1 winner from 56 runners, and it was 5/1. Again, they average around 1 in 16.