How do you mark a square cut line by hand?

Finish Carpentry
Miter Saw

A square line drawn by eye follows your hand, not the blade, which is why hand-marked cuts drift on wide stock. Fixing the length on the M1 Caliber and tracing the saw's own shadow line puts the mark where the blade will actually travel. It checks out against a square afterwards because it was never set by eye in the first place.

Why hand-marked lines drift

A line drawn by eye follows your hand
M1 fixes the length first
Trace the saw's own shadow line
The mark is where the blade will travel
Checks out against a square afterwards

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Let two tools set it for you. Zero the M1 Caliber on the blade edge and slide to ten inches so the length is fixed, then switch on the saw's shadow light and trace along the projected line. The line lands square because it follows the blade's own shadow rather than your eye, and it checks out against a square afterwards.