If you're using homegrown hops, I would recommend saving them for the end of the boil. Since there's no real way of knowing what the actual AA% is, you might as well save them for where you get the most flavor/aroma but less of an effect on your IBU's.
FWIW - Otter Creek Stovepipe Porter is one of my favorite beers, and they use Nugget, Cascade and Willamette. Even though the Stovepipe is a lot roastier than what you're going for here, I think your hop selection will be very good in a porter. The Chinook has a fairly aggressive bite when used for bittering, so just be aware of that.