Measuring CURVED monitor angles in a triple screen setup

Flat monitors have no mistery. Get a protractor and job done in seconds. But what about CURVED monitors? Do you:

A) Measure the angle of the sides monitor based on a flat imaginary line from the center of the center monitor to the center of the side monitor

OR

B) Measure the angle between the two connecting/touching edges of the side and center monitor.

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I'm doing A, but how many folks do B, and why they think this is the correct way.