rntzfootnotes.sty

\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{rntzfootnotes}[2018/07/02]

\DeclareOption*{\PassOptionsToPackage{\CurrentOption}{footmisc}}
\ProcessOptions\relax

\RequirePackage[marginal]{footmisc}
\AtBeginDocument{
  %% Comment out if norule is set. Also it uses ems, which is a bad idea if the
  %% font is scaled a lot. TODO: fix this. could I use \f@size & \dimexpr? Maybe
  %% should use exes, since it's a vertical measure.
  \setlength{\skip\footins}{1em plus .25em minus .2em}

  %% NB. In vanilla TeX, \footnotesep isn't actually a length, I think? At
  %% least, if I do \setlength\footnotesep{1em plus .25em minus .2em}, I end up
  %% with ``plus .25em minus .2em'' at the front of my document. UNLESS I
  %% \usepackage{mathtools}. I don't know why that fixes it. In any case, I
  %% don't want to have use mathtools, so instead I define \footnotesep in terms
  %% of \skip\footins, which seems to work.
  \setlength\footnotesep{\skip\footins}
}