I would not be worried about hard and fast ratios. Yes, I would do my best to estimate proper ratios, water to preserver. But this is a consumer-safe product intended to make your water potable.
Water that is not pure can potentially make you ill if you drink it before purification. My suggestion is to use multiple methods of purification whenever possible if there is ever any doubt about the condition of the water.
Storing pure, preserved water is a great move for anyone. We never know when something might happen that would contaminate our normal water supply.
If you've done it properly--stored good preserved water in approved sealed water containers, you're a step ahead of the game.
Still, if you want to be sure about that water, especially if it's been stored for a number of years, boiling it or any other water you have to consume is going to be the most prudent move to assure you and your family are safe. Using multiple levels of water purification is not a difficult thing to do.