When it comes to iced coffee, there is no definitive answer as to what the best ratio is. This is because everyone has different taste preferences and so what might be the perfect ratio for one person might not be the same for another. However, a good starting point would be to use a 1:1 ratio of coffee to water when brewing your coffee concentrate. Once you have your concentrate, you can then add more or less water (or milk) to taste. So, if you like your iced coffee on the weaker side, you would add more water (or milk) than if you prefer it to be stronger. Experiment until you find the perfect ratio for yourself!