a slight tangent, but I did heaps of research a few years back on pre post prices,

the conclusion was that overall the performance in terms of winners was almost exactly the same as the actual fav. even though in some cases the nags were different (if you see what I mean)

The important thing though is to always use the same source for the pre post fav, I use www.ozbet.com i.e. WA Tab Form (on line) whos' pre post prices are from commercial ratings converted to prices so I'm told.