You want to find the index of a substring within a string rather than the text before or after the substring.

<xsl:template name="index-of">
     <xsl:param name="input"/>
     <xsl:param name="substr"/>
<xsl:choose>
     <xsl:when test="contains($input, $substr)">
          <xsl:value-of select="string-length(substring-before($input, $substr))+1"/>
     </xsl:when>
     <xsl:otherwise>0</xsl:otherwise>
</xsl:choose>
</xsl:template>

The position of a substring within another string is simply the length of the string preceding it plus 1. If you are certain that the target string contains the substring, then you can simply use string-length(substring-before($value, $substr))+1. However, in general, you need a way to handle the case in which the substring is not present. Here, zero is chosen as an indication of this case, but you can use another value such as -1 or NaN.