Hey warlock what up,
I use the exact same stuff. I actually have a mini g/oz scale at home so I can measure it out by grams, makes it a whole lot easier for powder nutes/additives. Its takes 10 grams (five-six 1/2 tsp's)of PH down to bring my PH from 7.2-7.5 to about 5.8-6.0 sometimes more PH down is needed. My tap water starts at 7.3 PH and PPM is 550.
It seems like you wouldnt need to add much at all for it to go down, maybe teaspoon and half, do it a lil at a time 6.4 is pretty close for normal.
Listen to purp tho cus I am still a newbie so I might make mistake or 2 and to be honest Id ask purp if Im right at all before messing with anything haha.