The Eye-One Match software cannot measure the changed Brightness / Luminance, as long as the window of the control panel 'Displays' is still open. So please adjust the Brightness slider as desired, close the 'Displays' control panel, then wait a second until the eye-One match software has actualized the measurement values. If needed, open the 'Displays' control panel again, adjust Brightness again, close control panel and wait for actualized measurement values. Please repeat this procedure until you have reached the desired Luminance.
Follow up: I tried this method with some success. Was now able to drop luminance value to 160. It seems purchasing Color Eyes may the only way to get it down further.