If I did 9 it would be...
28g×18oz=504g
504÷220= 2.29GPW.
Of course this is if I average 2oz per plant. I have never done this strain, maybe I will get less per plant or maybe more. But I think 2 is a good guess. I will say that these Double Grapes are growing very well and there are 3 ahead of the game, but they all seem to be more Indica looking. I also have 2 taller phenos that are not flowering yet, they are all doing well.