[JohnM]

Dan, if you want to use a soundcard calibration file then uncheck the "Use left channel as calibration reference". The button to measure a cal file should remain disabled if "use left..." is selected, but it gets re-enabled after a level check (a bug, oops). If you are happy using the left channel as the reference then you don't need a cal file, just make a measurement with the loopback connected on left and right to make sure the result is suitably flat, then go ahead and connect your spl meter or mic on the right channel and carry on from there.