Betting exchanges charge commission on your net profit whereas traditional bookmakers do not because their commission has already been included in the calculation of the odds. So in order to do a comparison between the back odds offered on the exchange and a bookmaker's odds you have to deduct the commission. This calculator performs that task for you.
oBE - effective back odds
oB - back odds offered on the exchange
c - commission charged (as a fraction)
oLE - effective lay odds
oL - lay odds offered on the exchange