[Tex/LaTex] Math equation with cases and division signs

amsmathcasesequations

\begin{equation*}
\theta({P,\bar{Cl}}) =
\begin{cases}
S(P,\widehat{Cl}) + (S(P,\widehat{Cl}) \times  ((\bar{R}-\widehat{R})/\bar{R})) & \widehat{Cl} \textless \bar{Cl} \\
S(P,\widehat{Cl}) - (S(P,\widehat{Cl}) \times  ((\widehat{R}-\bar{R})/\bar{R})) & \widehat{Cl} \textgreater \bar{Cl}
\end{cases}
\label(eq: state}
\end{equation*}


How can I make it look more beautiful and legible?
I would like to push the division sign below so that it is more legible

any ideas are welcome

First of all, I would use displaystyle fractions. Next I would replace the non-extensible \bar with \widebar, borrowed from the mathx(it's in the mathabx bundle, without loading the mathabx package since it changes many symbols.As I'm changing symbols, I also replaced \widehat with mathx's version, with I named varwidehat to not interfere with defaults. Finally I changed Cl into a math operator (Cl), as I suspect it is so. For the moment it looks like the product of the two variables C and l. If I'm wrong, all you have to do is replacing back \Cl with Cl.

To prevent having too large vertical spacing with respect to the surrounding text, due to the fractions, I kill the first line fraction height with a \smash[t]{…} command, and the second line fraction depth with \smash[b]{…}.

\documentclass[11pt,a4paper]{article}

\usepackage{mathtools}

\DeclareMathOperator\Cl{Cl}

\DeclareFontFamily{U}{mathx}{\hyphenchar\font45}
\DeclareFontShape{U}{mathx}{m}{n}{
<-6> mathx5 <6-7> mathx6 <7-8> mathx7
<8-9> mathx8 <9-10> mathx9
<10-12> mathx10 <12-> mathx12
}{}
\DeclareSymbolFont{mathx}{U}{mathx}{m}{n}
\DeclareFontSubstitution{U}{mathx}{m}{n}

\DeclareMathAccent{\varwidehat}{0}{mathx}{"70}
\DeclareMathAccent{\widebar}{0}{mathx}{"73}

\begin{document}

\begin{equation*}
\theta({P,\widebar{\Cl}}) =
\begin{cases}
S(P,\varwidehat{\Cl}) + S(P,\varwidehat{\Cl}) \times \smash[t]{ \dfrac{\widebar{R}-\varwidehat{R}}{\widebar{R}}} &\qquad \varwidehat{\Cl} < \widebar{\Cl} \\[1.5ex]
S(P,\varwidehat{\Cl}) - S(P,\varwidehat{\Cl}) \times \smash[b]{\dfrac{\varwidehat{R}-\widebar{R}}{\widebar{R}}} &\qquad \varwidehat{\Cl} > \widebar{\Cl}
\end{cases}
\label{(eq: state}
\end{equation*}

\end{document}