I didn't use the extract, I bought dried bitter orange peel off ebay and used 55g as per the OP. Turned out spot on. The bag of peel was enough for 4 brews so I vac sealed it. If you go for the extract, the Brewery said they use Nielsen Massey dosed at 0.4ml per litre.
I haven't got Admiral so I'm subbing that for Citra, only because I've got a load of it
As the bittering hops are in for the full 60 mins I don't think changing them will make a big difference.