How do I set column widths in read/write rules?

Hello structure experts,

I am reading a set of DITA files into a specific FM template format, and there is one detail I cannot get to work properly. It should be possible to define column widths in a table, even if the DITA file does not include that information. But the Structure Application Developer Reference does not give the information on which string to pass to the column widths fm property. Does anyone out there have experience in doing this ?

Thanks

Jang