If it's helpful at all, for me, ph6.4 water at 600 ppm nute solution is lowered to 5.6 with 1ml of GH pH down.
Im using a 5gal too (1st attempt at hydro, so all i got is my limited experience), and it seems like, at this low of a ppm, i need to adjust ph like 2 or 3 days in a row after changing...