G'day Luckyboy,

Your culling method is quite sound except for one flaw-concentrating on horses that were in the first five place-getters last start. A particular horse may have run second last start but beaten by 6-7 lengths. In other words, something had to run a place behind the winner. That is why a punter should concentrate on runners which finished within 2 lengths of the winner no matter their finishing order. Also, that it is to contest a similar class of race today or preferably, a lower class.
The finishing order method is the mainstay of many mechanical systems and that is why most, if not all, are destined to fail in the medium to long term.

Cheers.