How to edit the length of a color box? - 1

\documentclass[letterpaper,11pt]{article}
\usepackage[left=0.9in, right=0.8in]{geometry}
\pagestyle{empty}
\raggedright
\def\bull{\vrule height 0.8ex width .7ex depth -.1ex }

\newcommand{\lineunder}{\vspace*{-8pt} \\ \hspace*{-18pt} \hrulefill \\} %command for the Line to be drawn
\usepackage[usenames,dvipsnames]{color}
\definecolor{mygrey}{gray}{.90}
%\newcommand{\headergrey}[1]{\hspace*{-18pt} \colorbox{mygrey}
%    {\begin{minipage}
%    {\textwidth}{\textsc{#1}}
%    \end{minipage}}\lineunder}

\newcommand{\headergrey}[1]{%
  \noindent\hspace*{-15pt}%
  \tabular{p{\dimexpr\linewidth-2\tabcolsep+15pt}}\cellcolor{mygrey}\textsc{#1}\\
  \hline\endtabular%
  \par\nobreak}%


\begin{document}
%\headergrey{Objective}
%    Seeking a Full Time position in the area of Blah blah blah Blah
\headergrey{Objective}
    Seeking a Full Time position in the area of Blah blah blah Blah
\end{document}