If it's unsweetened cocoa powder it will add bitterness to the finished beer. I've made a chocolate stout twice recently, and the second go around I cut back on the hops a bit because the bitterness was too much. I've never used cocoa in the boil, but I add it to my mash, about 8oz for a 10 gallon batch. The flavor is very very subtle, but the chocolate stands out well in the aroma. Android's calculations for 33 IBU seem right to me, not the 13.5 you have. I would probably drop to 1oz Fuggles @ 60min, 1oz Fuggles for 10min which would put you at about 21 IBUs. As to how much bitterness the cocoa powder will add, from my experience it's quite a bit, so you may want to add a little less than 1lb, because if it turns out to be too bitter there's really nothing you can do to fix it. On the other hand, if there isn't enough chocolate, you could always add some to the secondary, either in powder or syrup form(cocoa powder, sugar, water, vanilla extract).