This is the mathematical theoretical calculation assuming they have all equal chance of winning.
We are not looking at its positional ranking in the market or its price.

Lets assume a 10 horse race = 10% random chance of winning

Lets assume horse is in Race 4, Horse No.4 with 10 runners. = 10% random chance of winning.
Calculation
.10 x .10 = .01 x 100 = 1% chance of winning.

This is assuming they have all equal chance over 100 races.
But as we know , this is not the case in horse racing .
No.1 ranked in the market out performs No.10 ranked in the market , but at what price & that is the difference.

If one wished to create a lay plan around this idea , it might be a strong point to make the price range between say $3.80 - 11.00
You shouldn't get bit too often in this price band.

Staking to Bank
Bet 1% if $3.80-5.50
Bet .50% if 5.60-11.00



Cheers

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Cheers.