I don't find Magnum harsh at all, and is generally considered a pretty neutral-flavored, smooth, bittering hop. I think the recipe looks good. I would aim for around 40 IBUs. Alts are supposed to be bitter, and malty at the same time. This is achieved by using a light touch when it comes to flavor, and especially aroma hops - which you have at none. This helps to keep the beer pretty clean and crisp.