table column width relative to pagewidth?

[Rick Janda]

I can determine the width of a table column absolutely by inserting a
value at "Width" in the table layout dialog.  

Is there any way to do this relatively to the page width, that I can
make the column width 50% of the pagewidth by example, as I can do 
with figures?

I use Lyx 1.14

Thank you,
Rick Janda