A21 bus to the HK airport

Hi, I have 2 questions:-
1) I will be travelling from Kimberley Hotel. How far is this hotel to Nathan Road where the bus stop is?
2) where is the departure lounge for Singapore Airlinges if i caught the A21 bus to the airport. I will be leaving Saturday. My flight leaves at 6.30pm. What time should I leave to catch the bus? Many thanks!!