I understand the confusion, but the overround, or underround, are not applied - that is the situation regarding the field book. Which race is it that equal fair odds are on offer ?
The field book operates on 100% take out, calculated on price percentage : 7/2 is 22%,11/2 is 15%, 8/1 is 11% and so on for every price available. Every book on every race is overround i.e the take out is more than 100%. The only advantage the bettor has over the bookmaker is that the bookmaker has to bet in every race, the bettor doesn't - so, in a field of 8 or 10 runners, depending on the book takeout, taking 3 or 4 against the field for a total of less than 100% guarantees a profit - and, if that is a 10 runner field, that means that instead of having one horse running for you and nine for the bookmaker you have three or four running for you, which 60% or 70% of the field. This seems to me to be fairly basic.