I have been formatting my dissertation and one little problem is stucking me up. I used the following code to typeset an equation

\begin{align}
& R=\frac{P^2}{P+S'} \label{eqn:SCS}\\
&\mbox {where} \quad \mbox R = \mbox {Watershed Runoff} \notag\\
&\hspace{0.63in} \mbox P = \mbox{Rainfall} \notag\\
&\hspace{0.63in} \mbox S' = \mbox{Storage in the watershed $=\frac{1000}{CN}-10$ }\notag
\end{align}

My output requirement is such that: The equation should begin one tab space from the left margin The equation number should end at one tab space from the right margin

With the above code, I have the equation begin at the right place but not the numbering.

Any help will be extremely appreciated.

Thanks MP