Betfair trader’s next step is to equalize his profits, in other words to produce a green book or an all-green screen. All he has to do is divide the profit by the currently betting odds of the specific outcome of the betting event and lay that amount. The difference in case his trades resulted in a loss, is that he has to back (instead of lay) the amount which results from dividing the loss by the odds. I guess that would be a red book or an all-red screen.

Additionally there are free betting tools online which can help you calculate the amount to back or lay so that you have a green book. Chromaweb’s Arb Calculator is one of them which you can try for free for as long as you want.