siwelwerd wrote:Magnum is a very clean bittering hop, I use it extensively for this purpose. It will sub for the horizon just fine.
I would really try to get your hands on some Pale Chocolate, that lends some unique flavors that cannot be replicated by 350-450L chocolate malt.
+1 on this. If you REALLY want to add the subtle citrusy background bittering note, a small 45 or 30 minute addition of Cascade or Centennial (just a 1/4 oz or so) will help backfill that aspect -again, the key here is subtle. Try to make the IBUs match up by adjusting the Magnum, which is a Hallertaur derivative and that makes it mega clean and spicy. I love that hop.