performing calculations with variables / params in FO

Sample statement.

<fo:table-column column-width="{$SBWidth-$ImageWidthRender}"/>

This one place where I am using variables to define widths for FO table
columns

(I am primarily using FO tables and nested tables for laying out my
document)

QUESTION:

If I am specifying those variables as, 300mm and 200mm respectively I
suppose there will be a problem in trying to subtract them with the
units present.

Is there some way to specify a particular UNIT to be used all over so
that I can specify those values without the units appended (makes
subtraction easy)?

Is there a way to have this subtraction performed with the units
present,  without having to write intensive string parsing functions to
separate the values & units? 

FYI - We are using a licensed version of Apoc FO Renderer
(www.chive.com)
and .NET System.Xml.Xsl classes. ( derivative from MSXML 4.0)

Please do let me know what you think.

Thanks,

Abhishek Sanwal
HP - Houston Campus
abhishek.sanwal@xxxxxx


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