No - no guesswork! I used Firebug (a Firefox add-in) to see what margin would work.
I don't know why it put the gap there; did you change the width of the top level menu while designing it in the CSSMenuWriter interface? If you did, did the sub-levels look right in the preview?
I haven't always found the preview to be too reliable, even when I remember to press the 'update' button.