First let's get some simple conversions out the way:

Water
7L in + 7L out = 14L total

We're going to convert that to ml since it's a much smaller unit that we can make smaller batches with at home. So:

1L = 1000 ml therefore 14L = 14,000 ml

Now that we've got the water down let's get to the coffee. We use one full bag for all that water which is equal to 5 pounds. Let's convert that to grams since again it's a much smaller unit that we can use at home

1 lb ≈ 454 grams
5 lbs * 454 grams = 2,270 grams
5 lbs ≈ 2,270 grams

So now we have a water to coffee ratio of approximately
14,000:2,270

Let's simplify that by dividing each side by the amount of coffee so that we can see how many grams to into each ml of water

2,270 / 2,270 = 1 (duh)

14,000 / 2,270 ≈ 6.17 (it's a much larger decimal but let's just use two decimal places)

So now we know that for every gram of coffee there should be approximately 6.17ml of water.

Now when we dilute it with the chilled water, that's simple as that's just the same amount of water as there is concentrate.

So there we have it. A water to coffee ratio of approximately 6.17:1 is how you can make cold brew the Starbucks way at home. Do with this information what you will. If anyone sees any errors in my math or anything let me know and I can fix it up.