It's pretty clear that the original recipe was absurdly over the top at almost 100 IBU........ but the substitutions should produce a nice ale at about 60 IBU. Why magnum was added when it was I don't understand. True IPAs in the original sense should be in the 40's to 50's. Above 60 I call it an AABA....(Absurd American Bitter Ale)
60 IBUs is pretty bitter for many people's pallette, but I tend to brew up around 55 normally, so I would not find your brew outrageous. Amarillo is a flavor and aroma hop, and Magnum is a bittering hop as a rule. Using Amarillo and boiling 60 minutes, is an absurd waste. A more rational use of hops would be to use magnum at 60 minutes for bittering..... 3/4 ounce, and an ounce of Amarillo at 10 minutes or an ounce of Magnum for 60, and an ounce of amarillo at flame out or whirlpool, and then another for dry hop. This would put you in the mid to high 50's , and lots of flavor and aroma.
The important thing here is to plug your numbers into some software........... I like Brewer's Friend, which is free to use on line, and very easy to use. The other thing to consider is finding a complete description of the various hops, print it out, and take it with you when you go shopping. There is an IOS and I think an Android version of Brewer's Friend, and it's cheap........ take your phone or tablet and your hop description chart, and design your brew right there in the store based on what they have on hand.
H.W.