I didn't write the demo that 'solve' your problem, it only showed how to do text centering. Just as you said, the width of the first column will stay as what it is and only the second one will be resized.
The Label1 is for centering text on the form, Label2 is between the left edge of the form and the separator (TShape) and Label3 is between the separator and the right edge of the form. The calculation is relatively easy to understand, you can modify it to match your case.
Yes, I use Linux.