% monoref -- tuning the reserved slot widths.
%
% Forward values reserve a font-relative width from a template; at flush
% the slot collapses to the value's natural width and the line is
% re-justified, so short values never leave a gap. The template only has
% to be wide enough to guide the FIRST line break; make it as wide as the
% widest forward value you expect.
%
% Here a document with many pages (wide page numbers) and deep section
% numbers widens both templates. \monorefslotwidth would override both
% with a single fixed length instead.
% Compile ONCE with LuaLaTeX:  lualatex 06-custom-slot-width.tex
\documentclass{article}
\usepackage{lipsum}
\usepackage{monoref}

\renewcommand\monorefreftemplate{0.0.0}  % up to subsubsection numbers
\renewcommand\monorefpagetemplate{0000}  % up to four-digit pages
% Alternatively: \setlength\monorefslotwidth{3em}

\begin{document}

\section{Introduction}\label{sec:intro}
Forward reference to the deep dive in Section~\ref{ssub:deep} on
page~\pageref{ssub:deep}. Total pages: \lastpage.
\lipsum[1-3]

\section{Body}\label{sec:body}
\subsection{Part}\label{sub:part}
\subsubsection{Deep dive}\label{ssub:deep}
This is Section~\ref{ssub:deep}.
\lipsum[1-2]

\end{document}