Revit wants you to 'Tab' select the Ref Plane inside the family. Now you can use the 'move' command to move that instance element accurately. The process again - you have a family with an instance parameter. Use Tab to select the Ref Plane associated with the instance parameter. Now hit 'Move' (or your Keyboard Shortcut :-), and you can put that ref plane exactly where you want it.
Got this great tip from Aaron Rumple: