yah, there is a hard wired 15% setting for the column widths (left/right)
in templates/default/left_column.tpl you could replace {$left_width} with 120..
then the problem, which you are running into is the $body_width value (which is either 85% for 1 column or 70% if you have both columns turned on)..
if you have a fixed width forum, then just replace {$body_width} with another fixed number or percentage..
...
now if you have a right column issue too, then there is also a right_column.tpl file that can be adjusted..
i personally just remove all the $xxx_width values from the tpls and let the CSS classes take over. i also have a fixed width forum and only ONE column turned on, so everything is known and there is no guessing values based upon number of enabled columns..
your choice
