Hi William,
The criteria for images to be selected as radiometric calibration panel is it should have the tag of calibration image. It runs the test on the first 3 and last 3 sets of images. If one of the candidates is successful, the result is used and the remaining candidates are not checked anymore.
Pix4D will then check for overexposure at step 3. and then throws an error or proceeds. Thus it is best to choose the best set of images manually. We might have the automated feature of detecting the best set when the processing starts in the future.