I know how to Greenup against an event using the lay side of the market but i need the formula to hedge on the back side against the other opponent after i have already placed the first bet.

If i use a dutch style of betting this requires me to adjust the bet on both teams so i can make a profit, but what if i can't alter my bet on the other team if say i had them in a parlay bet of 3 teams and the first 2 teams have won and i want to hedge against the other team but was not able to use betfair for a lay bet i only had a back option.