TLB - How do I...

There is no way to set the buttons width exactly as toolbar width. The option defines the maximum width only, the real button width depends of the button's text length.

You can increase the value: TLB Settings->Metrics->Button margins->Width
This value will always added to the left and right of the every button.

Also try to option TLB Settings->Toolbar->Default alignment. If you set this option into Centre (and if the option Same width for all buttons with text is turned on), the buttons will be centered on the toolbar.