I've generally read that the ASA rating for my XL2 should be 320; but is there a way for formulating that accurately? I've read bits alluding to the use of a waveform monitor for that, but I'm not sure I completely understand.
Get yourself an 18% grey card. Light it flat and even, then put your camera on sticks in front of it. Zoom in so the card fills the frame, and turn on the auto exposure. Hopefully, your camera's built in Reflected light meter is calibrated to 18% grey. Take a note of what stop the auto exposure sets.
Then take an incident reading with your meter from the cards' position. Then adjust the ASA setting up or down so that the meter matches the stop on the camera. This is your camera's 'ASA'
It's only an approximation, but it will be close enough for broad judgements.