I like to base my systems on sound principles common to all winners ( statistically based from Malcolm Knowles research.
That way they are less likely to fall over.

I have always felt that any sound system should be based around CLASS /WEIGHT /FIELD STRENGTH ratings( FS available through the WIZARD online.)

As a matter of simplicity and also "at a glance stuff" is the FREE ratings on this site.
If one based their OWN criteria on the top 4 asessed horses ( which are PROFESSIONALLY asessed then one could do very well indeed.

4 things come immediately to mind such as:

Do any lead or race on the pace?

Are they in the same class or dropping in class ?

Is a senior rider or good apprentice riding today.?

Is the available price(s) enough to make a profit whichever horse wins.
i.e 2 horses left out of the top 4 then MINIMUM price I need is $3.
1 horse left then I need MINIMUM $2.

Cheers.
darky.