[xsl] Using variables in xpath expression

["Joseph L. Casale jcasale@xxxxxxxxxxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>]

I have several templates with similar code to the following:

  <xsl:template match="wix:Component[wix:File[substring(@Source,
string-length(@Source) - 7) = 'MyString']]">

I want to substitute 'MyString' with a variable and the 7 with a
"string-length($var) -1"
however the processor complains about the usage of a variable. This is xsl 1
with msxsl.

Is there any way around this so I can avoid hardcoding the string?

Thanks,
jlc