DOCBOOK-APPS: How can I customize the column width in a table?

[Giuseppe Greco <giuseppe dot greco at fantastic dot com>]

Hi,

I would like to customize the column width in a table.
Reading the DocBook Guide, I understood that I have to
use a span. OK, but for me it is unclear how...

The default column width is OK for HTML output, since it
is adjusted to the longer string;
However, on printed output, the column widht is based on
the page size and the number of columns (all the columns
have the same width).

It would be possible to say:

first column 20 % of the page width,
second column 30 % of hte page width, and
third column the rest?

Thanks.
Giuseppe