Hi,
A special variable will be added in the next program update.
But right now you can calculate this value. If you placed more back bets than lay bets on a selection, e.g. if you backed first, then it'll be:
(back_payout-lay_payout)/lay_price - back_matched
If you placed more lay bets than back bets, then it is:
lay_matched - (lay_payout-back_payout)/back_price