From a post on the groff list by Gunnar Ritter

You can set table column widths as a percentage of the total line width:

 lw(\n(.lu/10u) lw(\n(.lu*9u/10u).

This is possible because the content of w() is interpreted by troff, not by tbl. Unfortunately it only works with GNU tbl and Heirloom tbl because traditional tbl variants have a limit of 10 characters inside w(). A workaround to make this fully portable is:

 .nr %1 \n(.lu/10u
 .nr %9 \n(.lu*9u/10u
 lw(\n(%1u) lw(\n(%9u).

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