If your appointment is on May 12th, then you should be able to upload that day, via Webform the document issued by the medical center, confirming that you did the medical. So, you will be well within the 30 days time frame.
There is one line in my remedical instructions :
" Once your medical examination has been completed the Panel Physician will submit medical results to IRCC for assessment. To obtain a copy of your Immigration Medical Examination please ask the panel physician at the time of your appointment. "
Does it means I need not to submit any webform after the medical exam? Panel Physician will itself do it