Why must I move frame to center it?

["John R. Culleton" <john@wexfordpress.com>]

I am using some old code that Hans provided to create a pretty frame around 
some text, for purposes of a "Part"  header page. Here is my current test 
file:
--------------------------------------
\setupoutput[pdftex]
\definepapersize[bok][width=6.0in,height=9.0in]
\setuppapersize[bok][letter]
\setuplayout[marking=on,margin=.5in,textwidth=4.5in,width=middle,location=middle]
\setupcolors[state=start]
\starttext
%\startstandardmakeup
\noheaderandfooterlines
\startuseMPgraphic{bg}
    StartPage ;
      path p ; p := Page enlarged  -10mm ;
      path q ; q := Page enlarged -15mm ;
      drawoptions(withcolor .85white) ;
      fill reverse topboundary    q -- topboundary    p -- cycle ;
      fill reverse bottomboundary q -- bottomboundary p -- cycle ;
      drawoptions(withcolor .65white) ;
      fill reverse leftboundary   q -- leftboundary   p -- cycle ;
      fill reverse rightboundary  q -- rightboundary  p -- cycle ;
    StopPage ;
\stopuseMPgraphic
\vglue -1.55in  %???
\hglue -.75in    %???     
\useMPgraphic{bg}
\vglue -6.5in
\hglue .2in
\centerline{\bf Here is some text}
%\stopstandardmakeup
\stoptext
-------------------------------------------------

Now here is my question: why must I use the negative \vglue and \hglue lines 
(arrived at by experimentation) to place the frame properly on the page? Did 
I mess something up?
-- 
John Culleton
Able Indexing and Typesetting
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http://wexfordpress.com