How do bookmakers calculate the percentage when framing a market for a horse race?

I think the standard percentage is 118 but if they are uncertain, like in early betting the percentage might be 140 which means that every horse is under the odds. If they want people to bet the percentage might be 108 which means nearly every horse is over the odds which gives great value for the punter. They sometimes drop the percentage if there is not enough action in the betting ring to entice punters to bet.How do bookmakers calculate the percentage when framing a market for a horse race?
On average they have about 2% per horse to create an 'over-round' book. Its not an exact science, but generally to more runners the greater percentage. They will 'make their book' with early prices, then keep adjusting them according to where the betting money is going, right up to the last seconds.



In a perfect world the book would always be 100%; that is, that if you add up the probability suggested by the odds for every runner, then they would add up to 100%. The difference between 100% and the actual total is their over-round, or target margin. Some races they will win, some they will lose, but provided the book is %26gt;100% then in the long run they'll make margin.How do bookmakers calculate the percentage when framing a market for a horse race?
i think they take the last wagering odds...

EXAMPLE.....IF THE HORSE WON AT 8-1 AND WAS STEPPING UP

IN CLASS..I THINK THE ODDS WOULD BE....12-1

IF THE HORSE HAD LOST AT ODDS OF 8-1 AND WAS DROPPING

IN CLASS THEN I WOULD SET THE ODDS AT 6-1.....