I am no L&T expert, but I take this passage to be mockery, not fact. Sir W says that he only has 9 grey hairs of a side, meaning not much hair around the sides of the head, and nothing but a dab of powder at top, meaning that the ONLY thing on the top of the head was powder (no hair). So he's making fun of the Admiral's baldness, and surely exaggerating it for his own amusement. I'm sure the neighborhood Rite Aid :) in Miss Austen's time didn't have 20 shades of face powder to choose from, so if the Admiral did wear powder on his head, then it might have been obvious. But I no means take it as fact that the Admiral WAS wearing powder--Sir W is just making fun of the fact that he has no hair up there. He fails to recognize that not everyone is as concerned about their appearance as he is, and that it may not be a failing to not be concerned about it.