There is not a way to do it with the GUI. Backup your XML file, and open it with Notepad ++. From there you can do a search and replace.
There's also the alternatives of setting the max brightness on the controller output ( I set my Falcon to 80%)
Also you can set the brightness curve on the models in xLights which is a global setting.
For the XML file, you'll need to do it manually.
Good luck!
-Steve