I would go with a drop, I use 10-15 ml to drop my 15 gal reservoir 1 point. So I guesstimate around .7ml per gallon...5ml is a teaspoon. Distilled water is better than R.O water period. You need to add cal-mag to both.

Perfect distilled water in a perfect world has a PH of 7 there is many variables though...normally it will be more acidic from picking up atmospheric gases I would say its normally closer to 6 than 7.

I use GH PH down BTW....but I still would wait. If you dont id bet on more harm than good.