DOCBOOK: TermLength on VariableList

I've been playing around with variablelist a bit, and somewhat with the
output that I'm creating from it.  We have quite a few documents in our
collection where variablelist is the correct markup, but people have
been reluctant to use it because the output has a lot of white space.
(Still haven't convinced them that this is the "wrong" reason to change
what markup they're using)  
I stumbled across the "TermLength" attribute in the Duck Book, but it's
a little vague as to how it's used.  Searching through the stylesheets,
we found that the "default" that's in there is "20".  So, uhm, what does
"20" mean, and what should I specify for TermLength in a list where all
of my terms are quite short?  Thanks,
    Greg

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