However the bookmakers ' profit margins mean the figure is usually above 100 % .
Over-round : In theory a betting book should be 100 % , so a toss of a coin would be even money heads , even money tails .
The bookmakers ' profit margins mean the figure is usually above 100 % .
Over-round : In theory a betting book should be 100 % , so a toss of a coin would be even money heads , even money tails .