Yes, this answer is a couple years behind. Some manufacturers measure overall outside to outside others center of the tube to center of the tube.(for ease of measuring, center to center is the same as inside edge of one to outside edge of the other) Generally measured at the tips as this provides the easiest point to get an accurate repeatable measurement. The width you choose is based on largely on personal experience but as a general starting point have another person measure the width between the boney protrusions at the outer front of your shoulders(where the clavicle and acromion meet) then add about 2cm for bars measured center to center, or 4cm if they are measured outside to outside.
I've heard that for general comfort, having bars slightly too narrow can be more comfortable than too wide because your chest and upper back must do more work to bridge the gap of being too wide. While too narrow could possibly slightly restrict maximum breathing capacity and also cause elbow-knee interference. However issues with being too wide or narrow also depend on other factors like reach, bar/drop height, leg length ratio, etc.
Overall I think folks make all the wrong fuss over these sort of measurements, the optimums are rarely steep peaks on a graph, most often the optimum is just the middle of a nice wide gentle curve with plenty of margin for error.(ie cranks for the xs to xl frame range should span about 150 to 185[~25% or people 5'>6'4"] based on femur lengths and knee bending, but good luck finding anything outside of 170-175[~3%]. Meanwhile people will worry for days over 170 vs 172.5[~1%])
I mean my motorcycle has 80cm handle bars and my mountain bike 58cm my old roadbike 40.5cm, half that of my motorcycle.(I did find this a little bit cramped for my body but it never caused any notable issues) and other than the shock of transitioning between them if it has been a few months, they all work well enough.