There is no such ratio. You have to adjust it by taste.
First, there is no single physical variable which can be considered to be the "strength of flavor". Making a stock will extract thousands of different compounds from the bones and vegetables, in varying ratios. It's practically impossible to measure these concentrations, much less to map some combination of them to a subjectively perceived flavor strength.
Second, there is no single "strength" of commercial stocks. Some stocks will taste stronger than others. And since taste is subjective, it may even happen that not everybody agrees that stock A is stronger than stock B.
Third, even if you have a commercial stock with a flavor you like, there is no guarantee you can imitate it with dilution. They may be processing their stock differently from you, or may be using different ingredients.
But just because taste is subjective, it doesn't mean it isn't real. Your tongue is a great instrument for cooking - so you should simply take your stock and slowly dilute it (or reduce it!) to the concentration you need.
See also Optimum Bone To Water Ratio For Pork and Beef Stocks for a question from somebody who basically wanted the same as you, but framed their question a tad differently. The answer is still the same - adjust as much as you need, and not by some predefined amount of water.