If you have magnum, I would replace the two 60 minute charges with the appropriate amount of magnum to hit the bittering target. If you're ordering the kit then I'd just use what comes with it. You won't notice any difference. Magnum is just a higher AA hop that's used for bittering and takes quite a bit less. If you don't have it you'd have to buy an ounce minimum and you probably wouldn't use half of it in a 5 gallon batch.