hi weed
I think you can get over 18 but not sure avbout the height they will get than, coz u perfectly know that innft the plants grow fasters than any other sistems...
so i'm only 70% sure about that...!
maybe this should be better 16, but i really would like u to get the best of it!
ok so lets say 16 first and let see what happen and look whats going on
Change ur feeding program for just and only pure flowering nutrients...no more nitrogen almost the less that u can...