Anyone tried or has experience using Food Grade Phosphoric Acid as a PH Down?

Its 100% good. You did not need food grade but that's a nice touch. I also use a 85% ph down that is comprised of only phosphoric acid. I have the main bottle in the closet stored upright and I fill up a little tincture dropper bttl for my main tent and add in drops at a time. about 5 drops per gallon of water to get it low 6.0 and about 2-3 drops to get it aorund 6.5 for my tap water every place is going to be different based on mineral content of the water. You don't need to dilute it further with distilled as it just means you need to use more, but that could offer a bit better control if your working with small volumes of water, so the option is there but I think its not needed. Good Luck!