There is no from-ancestors function i know of.
Whats wrong with:

<xsl:if test="@att='checkThis'">
  <xsl:choose>
    <xsl:when test="count(ancestor::*)=count(following::childTag[1]/ancestor::*)">
      <!-- do nothing since this is the right depth -->
    </xsl:when>
    <xsl:otherwise>
      . . . error message . . .
    </xsl:otherwise>
  </xsl:choose>
</xsl:if>


This will likely also give an error message if there is no
following::childTag, so you may also want to test for this
condition.

HTH
J.Pietschmann

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