Just to offer an alternatively solution. I wouldn't use any lines to box the equations, especially as this is a "Table of Formulae", which should have a similar style to the rest of the tables of contents. It is also likely that the table will go over a page so rather use a longtable. It is also a good idea to include the page reference where the equation was firstly described.
You should have the notation or lists of symbols preceding such a table,

Followed by a pagebreak and the Table of Formulas

Here is the minimal,
\documentclass[crown]{octavo}
\usepackage{longtable,fancyhdr}
\usepackage{amsmath}[2000/07/18]
\usepackage{amssymb}[2002/01/22]
\newcommand{\DittoMark}{``}%''
\newcommand{\Ditto}{\quad\DittoMark\quad}
\newcommand{\Headings}[1]{\textbf{\small#1}}
\begin{document}
\chapter{General}
\label{aref}%
\[e= \frac{2}{a}
\sqrt{s(s-a)(s-b)(s-c)} \]
\chapter{TABLE OF FORMULAS.}
\markboth{\Headings{PLANE GEOMETRY.}}{\Headings{TABLE OF FORMULAS.}}%
\subsection{PLANE FIGURES.}
\subsection{NOTATION.}
\begin{tabular}{r@{~}c@{~}l}
$P$ &=& perimeter. \\
$h$ &=& altitude. \\
$b$ &=& lower base. \\
$b'$ &=& upper base. \\
$R$ &=& radius of circle. \\
$D$ &=& diameter of circle. \\
$C$ &=& circumference of circle. \\
$r$ &=& apothem of regular polygon. \\
$a$, $b$, $c$ &=& sides of triangle. \\
$s$ &=& \( \frac{1}{2}(a+b+c) \). \\
$p$ &=& perpendicular of triangle. \\
$m,n$ &=& segments of third side of triangle adjacent to \\
&& sides $b$ and $a$, respectively. \\
$S$ &=& area. \\
$\pi$ &=& 3.1416.
\end{tabular}
\newpage
\subsection{FORMULAS.}
\noindent\begin{longtable}{lr@{~}c@{~}l@{\qquad}r}
\multicolumn{5}{l}{\hspace{-2ex}\textbf{Line Values.}} \\
\multicolumn{5}{r}{\tiny PAGE}\\
\multicolumn{4}{l}{Altitude of triangle on side $a$,} \\
& $h$ &=& \( \displaystyle \frac{2}{a}
\sqrt{s(s-a)(s-b)(s-c)} \) & \pageref{aref} \\
\multicolumn{4}{l}{Median of triangle on side $a$,} \\
& $m$ &=& \( \frac{1}{2} \sqrt{2(b^2+c^2) - a^2} \) & \pageref{aref} \\
\multicolumn{5}{l}{\hspace{-2ex}\textbf{Areas.}} \\
Rectangle, & $S$ &=& $b\times h$ & \pageref{aref} \\
Square, & $S$ &=& $b^2$ & \pageref{aref} \\
\end{longtable}
\end{document}