I agree with Sean on that recipe...but hey, if you want it roasty to the point of acrid, then go for it. I had a nice American Porter last night and the brewer at that brewery indicated that he used more modest levels of the dark malts. It was dark and slightly roasty with a gentle caramel fullness that balanced nicely and made it eminently drinkable...but that might be too mellow for what you are after. Definitely water dependent - he used a little baking soda in his RO water with very very
light sulfates.
Cheers