I have just this situation :

<xsl:template match="/">
<xsl:for-each select="/root" >
<xsl:for-each select="my_node" >
<xsl:for-each select="my_node_parallel" >

<!--I Create Variable and calculate value for totale_finale-->
<--!I need to order using totale_finale value -->

<xsl:value-of select="format-number($totale_finale,'#.##')" />

<xsl:for-each select="/root/my_node/my_node_parallel">
 <xsl:sort select="... same as the select for $totale_finale...."/>
 <xsl:variable name="total_finale" select="..........."/>

cheers
andrew