I agree about the dogs breakfast. A class 3 in Newcastle or Mornington is a lot different to a C3 further afield. The 2 horses going up and down in class mentioned above would depend on number of runs for each [average prize-money per starts].

I find the easiest way to determine class across all race tracks is to use the prize money as an indication of class. A $12k C3 is useually better than a $5k C6 elsewhere. It doesn't always pan out but in a dog's breakfast situation, it's often a better guide than anything else. The up/down in weight mentioned by Chrome is also a good general rule of thumb..