I don't think a 15-20 minute addition is going to add half the bittering of a 60 minute one. 20 minutes down to 5 or so is for flavor,5 or less for aroma. Dry hopping is for aroma,which is linked to our sense of taste. But doesn't add flavor,only adds to our perception of it as a whole.
I like 20 & 10 minute additions myself,beyond the bittering 60 minute one if I'm doing one. But that Cooper's can def won't need a bittering addition.
You'll get about 6.4%ABV out of a 1.056 OG by Cooper's formula of (OG-FG)/7.46 + .5=ABV% with an FG of 1.012. I also used 6oz of hops in my IPA. 4.5oz of 3 hops in the boil,the remaining 1.5oz as a 1 week dry hop.
Everything works if ya let it-Roady(meatloaf)