tanking membrane about 1 sq metre (Bevendean BN2)

I have about 20% of the pot left- full pot covers 7.5 sq metres so think there is a good 1 left. its the rubber liquid you paint onto concrete to seal it --- before you put other flooring down. Its for those times where you are not sure if a damp proof membrane is in the concrete. Dont want it to go to waste as v expensive product