When you see the bat go forward start the watch. This is a split second before actual contact is made and more than a split second before the sound or "crack" of the bat. The time "a split second" it takes to click the watch will be very very close to when contact is made. When doing it this way, you will get true H-1B times, but you must remember to reset the stopwatch because it will be running everytime a swing starts.
H to 1B is contact until first base is touched. If you wait for the "crack" of the bat, you're too late.