<xsl:attribute name="content-width">
          <xsl:value-of select="$pageWidth - $PageMarginLeft - $PageMarginRight"/>


can't you instead do

 <xsl:attribute name="content-width">
          <xsl:value-of select="concat($pageWidth, ' - ', 
                $PageMarginLeft ' - ',
          $PageMarginRight)"/>


and leave the XSL FO expression handler (which is you, in this case)
do sort out the lengths?

5.8.12 Expression Value Conversions

David


 XSL-List info and archive:  http://www.mulberrytech.com/xsl/xsl-list