The reference material you attached emphasizes center-line measurements for a reason: they create a consistent mathematical system for U-channel, clamps, headers, fixed panels and doors.

What center-line means

Center-line measuring does not necessarily mean the center of the curb itself. It means the center-line of the glass thickness at the location being measured. Once that reference is established, deductions or add-ons can be applied according to the selected attachment system.

Imaginary lines

Imaginary lines are reference lines drawn conceptually through the enclosure to represent the position of the glass, curb, header or other construction feature. They let you measure complicated conditions—such as buttresses, neo-angle returns and headers—without losing track of the intended glass line.

Why this is important

Without a consistent reference system, measurements get mixed and the installer may accidentally combine one part measured to face-of-wall, another part to center-line, and another part to finished opening. That is how deductions become inconsistent.