Place node at right of and above of centered

Question

Is there a way to place a node in TikZ such that it is above/below a certain node and left/right of another, while being centered with the other corresponding node. In the MWE is below, I want to place that node centered both horizontally with node C and vertically with node A.

\documentclass[margin=0pt]{standalone}

\usepackage{lmodern}% font 
\usepackage{tikz}% drawing
% load libraries
\usetikzlibrary{shapes.geometric}
\usetikzlibrary{positioning}
\usetikzlibrary{calc}

\begin{document}
% additional styles
\tikzstyle{default}=[%
draw,
thick,
align=center,
%fill=white,
color=black,
%shape=diamond,
shape=rectangle,
minimum width=20mm,
minimum height=15mm,
text width=20mm,
]
\tikzstyle{start}=[%
draw,
thick,
align=center,
double,
%fill=white,
color=black,
%shape=diamond,
shape=circle,
%minimum width=10mm,
text width=10mm,
]

\begin{tikzpicture}
% guide drawing
%\draw [help lines, step=10mm] (-10,-10) grid (10,10);

% nodes
\node [default] (A)              {A};
\node [default] (B) [below=of A] {B};
\node [default] (C) [right=of B] {C};
\node [default] (D) [right=of C] {D};
\node [default] (E) [right=of D] {E};
\node [default] (F) [right=of E] {F}; 
\node [default] (G) [above=of F] {G};
\node [start]   (S) [right=of A]{Start};

% connections
\path [->, >=latex, thick]%
     (A) edge (B)
     (B) edge (C)
     (C) edge (D)
     (D) edge (E)
     (E) edge (F)
     (F) edge (G)
     (S) edge (A);

\end{tikzpicture}
\end{document}