Change blueprint number of columns?

Question

I'm using Compass for the first time. Instead of having 24 columns and 10px gutters, I'd like to have 12 columns and 20px gutters. If I put "width:960px" on my #container, the .showgrid class shows me a correct grid based on this :

$blueprint-grid-columns: 12;
$blueprint-container-size: 950px;
$blueprint-grid-margin: 20px;

I've put this in my _base.scss, and here is the result :

There are 12 columns for a total width of 960px. Right.

But if I apply "width:span(12)" or "include column(12)" on my container, it shows me only 6 columns for a total width of 480px (960/2) :

The widths are wrong and based on the original sizes : 24 columns.

Do you know why and how I could fix that?

And, subsidiary question : do you know how to remove the lines relative to line-height on .showgrid ?