You can do that and it would probably work.
The way I've seen it done, and I've done it myself before, is cold steep 4:1 water to coffee ratio for about 12 hours (put it in the fridge, covered), then add that to your beer. Tweak that ratio up/down depending on how much coffee flavor you want.
If you don't know how much coffee you need to use for you batch, then draw a pint or 2 of wort from the secondary, split it equally into 5 glasses, add X ounces of the cold coffee brew to glass #1, 2X to #2, 3X to #3 and so on. Then taste your 5 samples and decide which one you like the most.
Now you know the wort volume in each of the sample glasses and the volume of coffee brew you added to each, so just scale the volumes of the sample you liked best up to your batch size and add it in.