If you want to know more about back to lay bets then you can read this blog post: https://www.trickybet.net/betting/getting-started-betfair-trading-dobbing/
Low Risk = Risking 1 point to Win 0.5 points – The selection will need to trade at 67% or lower than the back price in running
Medium Risk = Risking 1 point to Win 1 point (A traditional Double or Bust method). The selection will need to trade at 50% or lower than the back price in running
High Risk = Risking 1 point to Win 2 points. The selection will need to trade at 33% or lower than the back price in running