I use GH's ph down for water that's 8.0 or so out of the tap, (phosphoric acid) and I use Fox Farms. When adjusting my nute water solution, I add the Fox Farms first, then add as much ph down as necessary to reach a 6.7 or 6.8 ph. When adjusting for plain watering (no nutes) it takes twice as much ph down.
Moral of the story: the nutes bring the ph down further.

Do you use the Fox Farms feeding schedule they have online? Tiz a pretty good chart, but I've gone half-strength with their Big Bloom (thru veg and bloom) for a while now with improved results.