IMO, FWH is for flavor. Also IMO, Magnum isn't a great flavor hop. I usually use whatever my 15-20 min. flavor addition would be for FWH. You don't have that in your recipe, so you'll kinda have to wing it. The way I figure what to do with FWH is to first decide how much hop flavor I want. I add enough FWH to get that flavor (usually 1 oz., sometimes up to 2) and look at how much bitterness that gives me. Although it's subjective, I count the bitterness addition form FWH to be the same as I'd get from a 20 min. addition. Then I add enough hops at 60 to get me to the total bitterness I want in the beer. Any other hop additions (the Amarillo and Cascade in your recipe) remain as they are. FWH does result in a very smooth bitterness IMO, but so do Magnum hops. Since your recipe doesn't seem to be calling for flavor hops and Magnums are so smooth on their own, this might not be the best recipe for FWH unless you want to add a flavor hop component to it.