Some XRFs claim the can accurately measure the comp when the head is not in contact with the sample, the one you used is obviously not one of them. I have never tried to XRF through a slab so I cannot provide a good opinion, but I would be very cautious in making interpretations with the algorithm showing such a high level of light elements (maybe Carbon, and oxygen from the plastic)
I think your math looks good but would still be hesitant to claim that is the calculated comp. Too bad you didn't get more readings