calculating the width of an unshow widget

[Timothy Flechtner <Timothy Flechtner tellabs com>]

hi all,
  i've been shuffling (ok, maybe staggering) around this wild, wacky
widget set , and am slowly beginning to figure out how much i don't yet
understand (yay!).  anyway, i was trying to account for the width of one
of my widgets.  i have a label inside a frame, the frame has border type
GTK_SHADOW_IN and a border width of 0, the label has an x_padding value
of 5.  my best guess for calculating the width desired by the frame is
(sorry its in gtkmm):

    string s = "Gtk+"
    Frame* f = manage( new Frame() );
    f->set_shadow_type(GTK_SHADOW_IN);
    f->set_border_width(0);
    Label* a_name = manage( new Label(def[i].get_name()) );
    a_name->set_padding(5, 5);
    Style* style = a_name->get_style();
    Gdk_Font font = style->get_font();

    // width should be a_name->get_style()->get_font().string_width(s) +
(2 * a_name->get_xpad())

but. . . it seems like i'm off by 4 (4 too little).  can anyone tell me
what accounts for the extra 4?

thanks!
-tim