  We develop a quasi-normal mode theory (QNMT) to calculate a system's
scattering $S$ matrix, simultaneously satisfying both energy conservation and
reciprocity even for a small truncated set of resonances. It is a practical
reduced-order (few-parameter) model based on the resonant frequencies and
constant mode-to-port coupling coefficients, easily computed from an
eigensolver without the need for QNM normalization. Furthermore, we show how
low-$Q$ modes can be separated into an effective slowly varying background
response $C$, giving an additional approximate formula for $S$, which is useful
to describe general Fano-resonant phenomena. We demonstrate our formulation for
both normal and fixed-angle oblique plane-wave incidence on various
electromagnetic metasurfaces.
