After the dive you open each usable frame in an image viewer — Fiji (ImageJ) is free and runs on anything, and any viewer with a ruler tool will do — and measure the width of each of the four charts, in pixels. Four numbers. Divide the widest by the narrowest and write down the ratio.
That is the frame check. Four charts in one plane, seen square, all read the same width. Tilt makes the far ones read narrower. A chart on a bump reads wider than its three neighbors. The ratio is your frame's own honest statement of how good the capture was, and it goes in the Entry next to the numbers the frame produced. There is no pass mark — what you can achieve depends on the substrate, the height and the camera, so a limit set anywhere else would describe somewhere else. Of your usable frames, you score the one with the smallest ratio. That is a rule, not a choice; anyone with the same three frames makes the same pick.
A diver who improves watches the ratio fall between Dive 1 and Dive 3. That is the feedback the course gives you on your hands.