Aruco determine position of a 2d point relative to marker

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I have a calibrated camera looking down upon a floor, where I have placed an aruco marker. I have detected the marker, and been able to draw axis on the rvec and tvec found in estimatePoseSingleMarker. I can create a grid of 'real world' object points from the center of the aruco that are projected onto the image such that the location of another object on the same surface can be identified as belonging within a particular grid tile.

Ideally though, what I would like to do is determine the actual distance from a given pixel known to be on the floor to the center of the Aruco. I would appreciate any understanding of the correct approach for this problem.