# Leaders in tocloft with minimum width

How can I redefine the leader in tocloft so that it is has a minimum length? The background is that I always want leaders in TOC to appear with at least two dots, forcing line breaks in titles if necessary. The example below is a bit silly. The problem arises more often with multicolumn TOCs.

\documentclass{book}

\usepackage{tocloft}

\cftsetpnumwidth{1em}
\renewcommand\cftdotsep{2}

\begin{document}
\tableofcontents

\chapter{This is a chapter}
\section{Medium length title}
\section{Very, very long title of a section bbb bbb bb bb bb bb b b b b b b b}
\section{Medium length title}

\end{document}


\renewcommand\cftsecleader{\leaders\hbox to1em{\hss.\hss}\hskip 2.9mm plus1fill} with 2.9mm changed to desired minimum length.