Ok,
a test,
the way to calculate according to the method that was shown, let's say ya have $100 bank on the day.

A 4.00 Pre Post with a $2.00 opener would equate to a 50 unit bet.
$6.00 with a 3.00 opener would be a $33.30 bet, ( 33) and so on

Both those 2 examples have the same rating of 200 each if it were in the same race. But ya have to do the entire field and get the best 3.
The trick is to wait and see what happens up to near the start of the race, then bet according to your opening calculations.
So for example if the $2.00 opener's price moved out to say 3 or 3.50 towards the start of the race, it's still a $50 bet.

Common sense does need to come into consideration on wet track though.
I know of a race one time where a outsider, non qualifier 1600M 6/1 pp, paid 10/1
It was a race 7, but saw on a race 2 of that same venue they had to go wide due to a wet patch on the track.
So I threw the rule book out and backed the 6/1 that went out to 9/1 in the ring, but saw a plunge on the screen, then it went back up again.
Smiles, the bookie saw it so he moved his price up to 10/1 thinking he get the attention off while the 2 favs ( near the barrier) were having it out.
It won, but was not a qualifier.