I'm at the same point you are. First grow, doing my flush before harvest. And I bought FinalFlush blueberry flavor. I just used it in my flush 2 days ago so not much info to give heh.

I know most people say you should get at least 1.5x the size of your pot in water. aka if you have a 5 gallon pot you should be using 7.5 gallons of water at least. Though its probably best I don't do that because even with three plants which i have it would take forever to get 22.5 gallons of clean, PH'd water heh.

i basically used 3-4 gallons (about 3-4x as much as i normally water them) and added the final flush. I hope that does the trick. Though to be on the safe side, I'll probably do another flush in a few days to be extra sure.