extjs grid - how to make column width 100%

The property width is a pixel width.

{
                xtype: 'grid',
                store: store,
                selModel: Ext.create('Ext.selection.CheckboxModel', {
                    mode: 'SINGLE'
                }),
                layout: 'fit',
                columns: [
                    {
                        text: "pum",
                        dataIndex: 'SRD_NAME_FL',
                        width: 400
                    }
                ],
                columnLines: true
            }

if i have only one column how can i make column width = 100% or if i have several columns - how to make last column stretch to end of grid?