Add an progress bar image based on page number

Question

I would like to add a "progress bar" to each page of my report. I have 25 images named pbar1.png to pbar25.png, as shown below, to be put on slides:

pbar1.png:

pbar12.png:

pbar25.png:

Because I have exactly 25 images, but my report may have more pages (or less), I map the page number to image number using a simple linear map:

I can do the calculations using calc. My problem is to determine how many pages there are. I can use lastpage package or just simply add a label at the end of the pages and call \pageref{lastpagenum}, but then when I run LaTeX on the file for the first time, the reference is undefined and I get an Arithmatic overflow error (as it appears in the denominator of the fraction). I tried using \getrefbykeydefault{lastpagenum}{page}{1400} from refcount to avoid getting zero, but that didn't help. I also tried \ifstrequal, which didn't help either.

Here is the MWE. Some comments are provided in the code.

\documentclass{article}

\newcounter{imagenum}
\newcounter{dummy}

\usepackage{calc}
\usepackage{graphicx}
\usepackage{refcount}
\usepackage{fancyhdr}
\usepackage{lipsum}
\pagestyle{fancy}
\lhead{
    % 1400 below can be replaced with any number number big enough
    \setcounter{dummy}{\getrefbykeydefault{lastpagenum}{page}{1400}-1}
    % To run without error, replace \thedummy below with 3 (which is 4-1)
    \setcounter{imagenum}{(\thepage-1)*\ratio{24 cm}{\thedummy cm} +1}
    %\includegraphics{../../../_Common/ProgressBar/pbar\theimagenum.png} 
    % includegraphics above clearly won't work. So simply just print the scaled image number
    \theimagenum
    }

\begin{document}
\lipsum[1]\newpage

\lipsum[1]\newpage

\lipsum[1]\newpage

\lipsum[1]

\label{lastpagenum}
\end{document}

Answer 1

You could simply check whether the lastpage reference (i.e. \r@lastpagenum) is defined (using the \ifdefined macro) before performing your calculations:

\makeatletter% required as some of the commands we will use contain "@" in their name
\lhead{
  % only perform the calculations if the "lastpagenum" reference is defined
  \ifdefined\r@lastpagenum%
    % 1400 below can be replaced with any number number big enough
    \setcounter{dummy}{\getrefbykeydefault{lastpagenum}{page}{1400}-1}%
    \setcounter{imagenum}{(\thepage-1)*\ratio{24 cm}{\thedummy cm} +1}%
    %\includegraphics{../../../_Common/ProgressBar/pbar\theimagenum.png} 
    \theimagenum%
  \fi%
}
\makeatother

The next step is to remove the use of the dummy counter, by getting / converting the last page reference into a number that can be used inside the calculations, using refcount's \getpagerefnumber:

\makeatletter% required as some of the commands we will use contain "@" in their name
\lhead{
  % only perform the calculations if the "lastpagenum" reference is defined
  \ifdefined\r@lastpagenum%
    \setcounter{imagenum}{(\thepage-1)*24/(\getpagerefnumber{lastpagenum}-1) +1}%
    %\includegraphics{../../../_Common/ProgressBar/pbar\theimagenum.png} 
    \theimagenum%
  \fi%
}
\makeatother