Here`s a way of calculating the overlays which seems to pan out for quinellas.

To get a reasonable approximation of the possible return ,
simple multiply the 2 dividends together, then divide by 2. this will give you an idea of what it should pay , most times it pays a little more than the figure you calculated.
You will see the odd time that it is way short of the calculation.

I have come accross an observation that amazes me , that is, how close the final Quin. Divs. are when using the newspaper
pre- post prices & I dont know why.

So what you can do with this info is calculate your various combinations & set your market price with that formulae ,with each couple , then calculate the possible divs. using the said formulae on the TAB prices.
Have the 2 markets side by side ,then bet the overs.
Sounds good in theory but getting the bet on in time is the hard bit.

There is a program called BetMaster ,which can do all these calculations for you using newspaper prices after you have punched them in , its amazing how close the final figures are.Ph:0411265603 for details.

Quinellas can be frustrating , the problem is the majority in the $15.00 less payouts that kill you , in the end.
BetMaster can stagger the bets , placing more on the short priced combinations & less on the wide price combinations , but even then , its hard to make a profit because at the end of the day you have to have the ability of selecting the end result of not one but 2 horses,
one seems to get one but not the other ,

there are some folk who have a real vibe for Quinellas & are good at it .
It beats me how they do it, because of the low dividends to percentage risk involved.

I hope this helps you in your quest for that "Fleece Of Gold at the end of the Earth"

__________________
Cheers.