Indentation on numbered paragraphs

Question

I'm trying to format my text according to the following rules:

1. From the second paragraph on, paragraphs should be numbered.
2. Paragraph numbers should be inside text area, aligned to left margin.
3. First line of paragraph text should be indented by 2.5cm (starting from margin, not from number)

So far I have achieved the following.

\documentclass[12pt,a4paper]{article}
\usepackage{indentfirst} %indent first paragraph
\usepackage{lipsum}

\setlength{\parindent}{2.5cm} %paragraphs should be indented by 2.5 cm from margin

\newcommand{\parnum}{\arabic{parcount}.}
\newcounter{parcount}
\newcommand\p{%
    \stepcounter{parcount}%
    \parnum \hspace{1em}%
}
\stepcounter{parcount} % start at number 2
\newenvironment{parnumbers}{%
   \setlength{\parindent}{0cm}
   \par%
   \everypar{\noindent \stepcounter{parcount}\parnum \hspace{2.3cm}}% NEED TO ADJUST. Paragraph text (not numbers) should be indented by 2.5cm from margin.
}{}

\begin{document}
\lipsum[1]
\begin{parnumbers}
\lipsum[2-15]
\end{parnumbers}
\end{document}

Instead of having a fixed \hspace after the paragraph counter, how do I insert 2.5cm minus the length of the paragraph number and the dot? For example, when I have double digits paragraph numbers, the fixed \hspace makes the text slightly more indented then it should be.

Any thoughts?
Thank you in advance.
Rossi

Answer 1

The numbox can be set into some \hbox of specified with.

\hbox to \boxindent {content\hfill} will fill the box and place a box of the relevant width into the beginning of the paragraph.

\documentclass[12pt,a4paper]{article}
\usepackage{indentfirst} %indent first paragraph
\usepackage{lipsum}

\newlength{\boxindent}
\setlength{\boxindent}{2.5cm}
\setlength{\parindent}{\boxindent} %paragraphs should be indented by 2.5 cm from margin

%\newcommand{\parnum}{\arabic{parcount}.}
\newcounter{parcount}
%\newcommand\p{%
%    \stepcounter{parcount}%
%    \parnum \hspace{1em}%
%}
\renewcommand{\theparcount}{\arabic{parcount}.}
\setcounter{parcount}{1}
\newenvironment{parnumbers}{%
   \setlength{\parindent}{0em}
   \par%
   \everypar{\hbox to \boxindent{\raggedright\stepcounter{parcount}\theparcount\hspace{1em}%
       \hfill}}
}{}

\begin{document}
\lipsum[1]
\begin{parnumbers}
\lipsum[2-20]
\end{parnumbers}
\end{document}

Answer 2

A referenceable variant, with a simple \makebox[2.5cm], and \hangindent\hangafter in case you want the left margin of the paragraphs to be 2.5cm right of the document left margin:

\documentclass[12pt, a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc} %\usepackage{indentfirst} %indent first paragraph
\usepackage{lipsum}

\setlength{\parindent}{2.5cm} %paragraphs should be indented by 2.5 cm from margin

\newcommand{\parnum}{\arabic{parcount}.}
\newcounter{parcount}
\stepcounter{parcount} % start at number 2
\newenvironment{parnumbers}{%
   \setlength{\parindent}{0cm}
   \par%
   \everypar{\noindent\hangindent=2.5cm\hangafter=1 \refstepcounter{parcount}\label{par-\arabic{parcount}}\makebox[2.5cm][l]{\arabic{parcount}.}}%
}{}

\usepackage[colorlinks]{hyperref} %

\begin{document}
\lipsum[1]

{\begin{parnumbers}
\lipsum[2-15]
\end{parnumbers}}

We see in §\,\ref{par-10}…

\end{document}