Basically.
The example I gave was
3 $10 horses (representing 30% of the market)
versus
a $1.2 horse and 2 others. ($1.2 represents about 85% of the market.) Add 2 $10 horses onto that and you are betting your 3 horses, but betting into a 105% market already. Guaranteed to lose.

A tab market is ~117%. Say there is 10 horses. You can take the first 8 horses in betting.
You have bet on 80% of the field.
But
Chances are you have bet to about 110% market.

I basically always convert prices to chances of winning. On average, this works fine. Although obviously you have individual horses being well over/under rated.