for a 5 gallon batch, I used 1/2 lb of black patent for my porter, (Extract + Steeping grains; 1lb C60, 1/2lb Chocolate, 1/2lb BP) and it came out with a nice roastyness. I don't know that I would use less than 1/4lb, and I would be leaning towards the whole 1/2lb.
Adding it after the mash is supposed to be a bit smoother, and is definitely good if you don't want to worry so much about mash pH. I haven't used that method, but Gordon Strong talks a lot about it in his book.