The binding is only used currently for FlowTemplate setup and not for cover factory. If using a FlowTemplate to define each item in the FlowControl it works on all views that utilize one. The binding (in beta 2 now that I corrected the logic) is an absolute value. So if ItemSpacing is set to (12,12) and SelectedItemSpacing is set to (16,16) the spacing of the item currently selected should be 16, not 28 (as it incorrectly was in beta 1)