How do we calculate Layers Edge ?

First let's look at Edge is ?

The predicted future gain or loss is called expectation or expected value and is the sum of the probability of each possible outcome of the experiment multiplied by its payoff (value). Thus, it represents the average amount one expects to win per bet if bets with identical odds are repeated many times.

http://en.wikipedia.org/wiki/Gaming_mathematics#Expectation_and_strategy

If we have historical data we can work out our edge as a percentage by (total profit / total risked) * 100

When we calculate the layers edge we can use any of the 3 formulas below :

Layers Edge = Total Profit / Total Liability * 100

Layers Edge = Backers edge / (Average Odds -1)

Layers Edge = ((Back strike Rate * Potential Average Profit)  - (Lay Strike Rate * (Average Lay Losing Odds - 1) ) ) / (Average Odds - 1)

All 3 formula's will produce the same result.