First off,I want clarify that the Cooper's cans make 5 imperial gallons,which at 23L,equals 6.072 US Standard gallons. Keep that in mind when brewing/adjusting them.
I've been mixing cooper's cans with Munton's plain DME's since my 2nd brew. I came up with the formula of 1 Cooper's can,1) 3lb bag plain Munton's DME,& minimum of 2oz hops per batch. I brew them to 6 US gallons generally,& get from 5.3-5.9%. I used that formula with 6oz of hops in my BuckIPA,& it came out to 57c per bottle.
I think combining the cooper's cans with DME & hops to def be a good formula for some very good ales. But not all of the series cans use their ale yeast. That depends on the series & the beer involved.
But I haven't followed the Cooper's directions since my 2nd brew. Check out my recipes for ideas & my way of using them.
Everything works if ya let it-Roady(meatloaf)