Stems are measured along the centerline of the extension, from the center of the bar to the centerline of the fork. With large angles it's best to measure from the side where the centerlines can best be referenced.
Adjustable stems present a problem because the centerline distance to the fork varies as the angle changes. I'd assume that the published dimension is with the stem hinge opened straight, because once they're flexed, too many variables enter the picture.
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