I know what Chrome was saying is correct:

Originally Posted by Chrome Prince
"Because you lose less betting on odds on favourites.
Longshots, you have to overcome 30% plus percentages against you versus 5% to 10% against."

But only as far as overall stats. go.

We don't bet 'overall', we bet 1 race at a time and in that situation, the percentages are meaningless, regardless of how many bets we have. Also, a $5 loss on an odds-on favorite is exactly the same $5 loss bet on any other runner. Every race bet is separate from any other bet. Their is no rule that says a longshot punter will lose more than an odds-on punter. It depends on the punter. If I back a horse at say $10, that's not because I think it is a 9/1 chance, it's because I think it's a 3/1 or 2/1 chance!

The logic is pretty logical actually, you have to find a 10% edge to make money as opposed to finding a 30%+ edge.
It's easier to find a smaller edge than a larger edge.

As crash says though, it's probably also easier to find a horse that is completely mispriced at longer odds.

How do you measure 'edge' in a race [especially 10%] accurately? It's only opinion isn't it or is there a 'true odds' handbook each race day I don't know about?

As you said, I have no trouble finding plenty of horses I'd judge [ in my opinion] have 30%+ edge in their odds. I can be out quite a bit and still be mostly on overs, but 10%? $2.70 instead of $3?
Flip a coin, heads your overs, tails your unders. That's about as accurate as anyone will ever get in an individual race before it starts.