I've never used beersmith but the Brewer's Friend online site is fairly user friendly.
I can't help very well with extract as I never brewed with it, going straight from kits to all grain but I can help a bit with the hops.
First thing is terminology, when discussing hops the times stated are the time remaining the boil, so your timings above would be:
15 g magnum at 60 min
15g comet at 45 min
30g Columbus at 30 min
This probably isn't going to get you what you're looking for, flavour and aroma mostly come from late hop additions, so anything boiled less than 15-20 mins. When I'm doing hoppy styles I do 3 additions at 15, 10 and 5 mins then I add enough hops at 60 mins to hit my target IBUs. I've never actually dry hopped but that will definitely help you to get the flavour your looking for.
Are there a couple of example beers you can give us so we know what you're aiming for? American IPA is a bit broad and could be anything from a crisp and bitter beer to a hazy juicy one, but I think you're going for more of the traditional non-hazy variety?
Gerryjo is right, we can't just talk about hops in terms of weight as the alpha acid percent also determines the bitterness you get. Flavour and aroma from late hops is more dependant on oil content but I don't tend to worry about that given I don't brew massively hoppy beers.