Measure distance from text to bottom margin

Question

I'm making some boxes for providing answers. The box macro takes the height of the box as an argument. I need to measure the remaining space from the line the box starts at, to the bottom of the page.

Consider the following MWE. The first box has the length I want, I need to provide the remaining length to the end of the page as an argument to the second box. How may I achieve this?

(Bonus question: why do I get Underfull \hbox (badness 10000) in paragraph error in this MWE?)

\documentclass[]{article}

\newcommand\mybox[1]{\vrule\leaders\vbox{\offinterlineskip\hrule width.1pt\vskip#1\hrule\kern-\dp\strutbox\hrule width0pt depth\dp\strutbox}\hfill\vrule\mbox{\vspace*{2em}}\\}

\begin{document}
    Please provide the answer in the box below \par\mybox{1in}

    Please provide the answer in the box below \par\mybox{4in}
\end{document}

Answer 1

No box requirements can go without a solution using tcolorbox!

tcolorbox provides height fill option to fill the remaining space in the page.

\documentclass[]{article}
\usepackage[most]{tcolorbox}
\newtcbox{example}[1][]{%
  enhanced jigsaw,
  colback=white, 
  colframe=black,
  boxrule=1pt,
  sharpish corners,
  arc=0mm,
  boxsep=0mm,
  left=\dimexpr\textwidth-2pt\relax,   %% 2pt= 2* boxrule width (1pt)
  right=0mm,
  %width=\textwidth,
  height fill,   %% <---- ths fills the remaining space
  #1,
}

\newcommand\mybox[1]{\vrule\leaders\vbox{\offinterlineskip\hrule width.1pt\vskip#1\hrule\kern-\dp\strutbox\hrule width0pt depth\dp\strutbox}\hfill\vrule\mbox{\vspace*{2em}}\\}

%\usepackage{showframe}
\begin{document}
    Please provide the answer in the box below \par\mybox{1in}

    Please provide the answer in the box below \par\example{\null}
\end{document}

Advantage here is your box can be made very fancy.