[nomercy400]

You are overlooking one major thing: From the perspective of the camera, the position of the moon relative to the earth matters for the number of pixels. For example, if the moon was directly between the earth and the camera, the earth-to-moon distance in pixels would be 0, but the real distance would still be ~384.400km.

You cannot use the earth-to-moon distance, unless you also know the exact position of the camera, earth and moon in the 3D plane.