Never been short changed by a taxi driver. They have often rounded down.
I once left a bag containing my passport and cash in a taxi. The driver tracked me down and the next morning drove from Tsing Yi to central to give it back.
Welcome to Hong Kong.
Another positive taxi driver story from today: dropped phone in car unnoticed on exiting; used Find My iPhone about an hour later. Driver called back and returned phone for cost of meter from his location ($100).