There is nothing underwater to measure a frame with, and a length or a color judged off a camera screen is not a measurement. Every number derived from an image 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 — each L with arms of equal length and width and a corner that reads square — and none may look foreshortened.
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. Center the frame on the four charts while you do it, so the lens's own distortion at one corner cannot fool you. It is not exact: a small tilt survives it, which is why the frame is measured for it afterwards rather than declared square in the water.
A diver under task load will skip it. Do not.