tabular - horizontal width

Hi all,

My question concerns the tabular environment.
I have a tabular with two columns and I want the full tabular to be
exactly 1.0\textwidth width (horizontal width equal to text around the
tabular).
I tried to give column one width "20 text%" and column two "80 text%",
but that does not work out, the sum is bigger than "100 text%". I assume
because borders/margins are not included in the column's width.
I also tried one column of "100 text%" which also is too wide (I didn't
forget \noindent).

Are those units documented somewhere? Like text%, line%, col%, etc.?
Can I solve this problem using a LaTeX argument?

Thanks.


%% LyX 1.3 created this file.  For more info, see http://www.lyx.org/.
%% Do not edit unless you really know what you are doing.
\documentclass[english]{article}
\usepackage[T1]{fontenc}
\usepackage[latin1]{inputenc}
\usepackage{array}

\makeatletter

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% LyX specific LaTeX commands.
%% Because html converters don't know tabularnewline
\providecommand{\tabularnewline}{\\}

\usepackage{babel}
\makeatother
\begin{document}
\noindent |\hfill{}|

\noindent a a a a a a a a a a a a a a a a a a a a a a a a a a a a
a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a
a a a a a a a a a a a a a a a a
% or something like \noindent |\hfill |

\noindent \begin{tabular}{|p{1.0\textwidth}|}
\hline
100 text\%\tabularnewline
\hline
\end{tabular}

% I tried to give a LaTeX argument in the LyX dialog
\noindent \begin{tabular}{|p{0.20\textwidth}@{\extracolsep{\fill}}}
\hline
20 text\%&
80 text\%\tabularnewline
\hline
\end{tabular}

\noindent \begin{tabular}{|p{0.20\columnwidth}|p{0.80\columnwidth}|}
\hline
20 col\%&
80 col\%\tabularnewline
\hline
\end{tabular}
\end{document}