That extra 30 is probably for the titlebar of that window. Try the following to commands. The first gives you information about the window itself, including borders/titlebar. The second gives you just the display area for that window.
//echo -ati2 * (xywh) $window(@windowname).x $window(@windowname).y $window(@windowname).w $window(@windowname).h
//echo -ati2 * (xywh) $window(@windowname).dx $window(@windowname).dy $window(@windowname).dw $window(@windowname).dh
