Hello Peter,
This calculation is only required if your device is operated within a certain distance of an human body.
At the time we did the certification of the ST25R3911B-DISCO it was around 20cm.
Since the ST25R3911B-DISCO is not intended to be operated in close proximity to the human body, those calculations where not be required.
I assume such calculation is more for devices like implants or device which are in close proximity to the human body.
The below link should help you proceeding with your calculation.
https://apps.fcc.gov/kdb/GetAttachment.html?id=f8IQgJxTTL5y0oRi0cpAuA%3D%3D&desc=447498%20D01%20General%20RF%20Exposure%20Guidance%20v06&tracking_number=20676
If you have any doubt I would recommend you to crosscheck with the accredited lab, which carries out your compliance measurements.
BR Travis