I think you have to set the three object variables for each item that you wish to be controlled by the detail level.   So for an object that will show in Low, Medium, and High Detail, you would need to set all three (750,751,752) all to true.   For an object to display only at Low, you would set 752 to True and 750, 751 to False.   For an object to display only at High you would set 750 to True and 751, 572 to False.   Try this on a simple object and see i