Instead of adding a button for double spacing, choose the LineSpacing
command from the All Commands category. This will give you a button with a
menu that displays 1.0, 1.5, 2.0, 2.5, 3.0, and More... (which opens the
Format | Paragraph dialog). For general use, however, it's easier to
remember these shortcuts:
Ctrl+1 = single spacing
Ctrl+5 = 1.5-line spacing
Ctrl+2 = double spacing