Grâce @touhami, j'ai résolu mon pépin : les décimales ont disparu. Mon code devient :
\documentclass[12pt, oneside, english, a4paper]{standalone}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[fulloldstyle,largesmallcaps,light]{kpfonts}
\usepackage{tikz}
\usetikzlibrary{calc}
\begin{document}
\thispagestyle{empty}
\begin{tikzpicture}[x = 0.03 mm, y = 0.03 mm]
\pgfmathparse{246/sin(4)}
\draw[red, ultra thin] (311,792) --+ (86:{246/sin(4)}) node[fill = white, midway] (n1) {\tiny \pgfmathparse{int(\pgfmathresult)} \pgfmathresult~mm} -- (802, 792) ;
\filldraw[fill=white, draw=black] (0, 0) rectangle (1500, 792) ;
%\draw[<->] (r1.center) -- (c1.center) node[fill = white, midway] (l1) {\tiny Abstand} ;
\end{tikzpicture}
\end{document}
