This Template is a self-reference and so is part of the Wikipedia project rather than the encyclopaedic content.
A border of 2px (1px width on each side) corresponds to a 5%. Therefore, with a 2px border — like on Wikipedia:Tutorial (Wrap-up and more info) — the width needs to be 95% for the table to fit within the screen. Since it's possible that some tables might not have a border, correction on the individual page could be made after substitution. Another way do a two column table is like this: