There is nothing underwater to measure with, and a length or a color judged off a camera screen is not a measurement. Every number in this course is computed afterwards, from the recorded frame.
What you do in the water is observe, and there is one observation that matters most:
The four charts must appear the same size and shape in the frame, and none may look like a trapezoid.
That is the whole square-to-substrate check. It is objective rather than a matter of taste, it takes a few seconds, and it works on a slope without your needing to know which way the slope runs — because the charts define the plane and your eye squares the camera to it. It is not exact: a small tilt survives it, which is why the frame is measured for tilt topside rather than declared square in the water.
A diver under task load will skip it. Do not.