Using Cross References to do this will give you a field. All fields can be updated any time by selecting them and pressing F9. To update all the fields in the entire document you just press Ctrl-A then F9.

It is an odd requirement to expect the period to be included in the cross-reference field instead of just adding it after the field. It can be done if you modify the cross-ref field code to add a specific number formatting. For example you could change
{REF _Ref22119258 \r \h}
to
{REF _Ref22119258 \r \h \# "#0."}

I have no idea on what you expect to happen if a new number 1 clause is added. Are you saying clauses B and C should be smart enough to automatically include a cross-ref to the new 1 as an exclusion but not if I add a new clause 6?

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Andrew Lockton
Chrysalis Design, Melbourne Australia