So the simple theory is to just back the best class horse 1x5 ew. in a race and we are forever in clover ?

So what are the rock solid rules for working out the above? Do we go on average prize money, SR or our or someone else's idea of a runners 'potential' ? Or, is it some sort of combo of the three [or more requirements] that has solid rules?
Except for those races where the Class runner is obvious, if there is no solid rules to define the class runner, we are back to making subjective decisions about class and which runner is the best class in a race.

These are serious questions, I'm not trying to take the micky here, just trying to define and get my head around what is on the surface a great idea, but below the surface seems very vague and loose.

Example of vague and loose: How could anyone say before the race [considering the standard of runners involved] MR711 Rewaaya
2nd ( $1.60plNSW) was the class horse of the race to back? When it comes to Stakes, Group and any other high class races with many classy horses running, we are back to good old fashioned personal preferences and tipping.

The Aust. stakes result only proves [finally] Rewaaya could hold his head up in that class of race. Before the race that was an unknown even to the trainer who said the race was it's big test of class. We can't make money on Class back-fitting.