How to code for a text string of unknown length to end at a specific point on the line?

I am coding a monograph template with 1-inch margins on the left and right of each page. The page header contains the title of the monograph. The header on lefthand, even-numbered pages (LE) and the header on righthand, odd-numbered pages (RO) are to be balanced: the text of the LE header is to start 0.5 inches in from the left margin, and the text of the RO header is to end 0.5 inches in from the right margin. In the MWE below, the LE header is correct but the RO is not—the text ends at the right margin, not 0.5 inches in from it. The title of an actual monograph using this template may be longer or shorter than the placeholder title, so the template doesn't know where the text in an RO header will begin. How does one code for a text string of indeterminate length to end at a specific point on the line? Compiled with XeLaTeX.

\documentclass[12pt, openany]{book}
\usepackage[letterpaper, margin=1in]{geometry}
\usepackage{fancyhdr}
   \pagestyle{fancy}
\fancypagestyle{monographstyle}{
  \fancyhf{} 
  \fancyhead[RO]{
    \begin{minipage}{\textwidth}
      \raggedleft
      \hspace*{-0.5in}
      {\fontsize{8pt}{10pt}\selectfont\sffamily\bfseries\
      The Placeholder Title of the Monograph}
    \end{minipage} }
  \fancyhead[LE]{
    \begin{minipage}{\textwidth}
      \raggedright
      \hspace*{0.5in}
      {\fontsize{8pt}{10pt}\selectfont\sffamily\bfseries\
      The Placeholder Title of the Monograph}
    \end{minipage} } }
\begin{document}
\pagestyle{monographstyle}
Lorem ipsum....
\newpage
Lorem ipsum....
\end{document}