Neutral ph is 7.0 so it seems your rain water is fairly close already. The roof runoff seems to be the best (maybe your containers caused the acidic rise in ph in them?). Mixing your 6.4 roof runoff rain water with your 7.8 (which is moving into being a base and out of neutral) tap water should be fine to use on your orchids in just about any combination.
Any true scientist here with more specific information for Dana? Please feel free to speak up and give her a more accurate analysis/ recommendation.