Central windows , i got just short of 900 L at the end , but i have tried it again since with the next lot of used resin and it didnt work at all , all i can think that happened is that i changed the original resin before i was due to , it might have been down to a dodgey reading on my tds meter !
Having said that though , i got 900 L from the bin experiment and i normally get 1250 L from fresh resin in the di at my normal tds reading , what i am saying is that there is no way that i took it out of the di after 350 L , so it must have lasted longer .
I have just got another bag of resin so im going to give it a go with unused resin just to see what happens , will let you know the results .
Rich P @ F