The rule of thumb is, try 1mg/lb at first...but you can safely also give 2mg/lb too.
I see a Vet Derm. - and they actually recommend 2mg/lb across the board.
Other antihists the Derm recommends for dogs are: Chlor Trimeton, Claritin, Zyrtec (the last 2 are nice bc they are 24hr pills, so you only give once a day).