  We develop a method to parameterize elastic-scattering phase-shifts for
charged nuclei, based on Pad\'e expansions of a simplified effective-range
function. The method is potential independent and the input is reduced to
experimental phase shifts and bound-state energies. It allows a simple
calculation of resonance properties and of asymptotic normalization constants
(ANCs) of subthreshold bound states. We analyze the $1^-$ and $2^+$ phase
shifts of the $^{12}$C$+\alpha$ system and extract the ANCs of the
corresponding bound states. For the $1^-$ state, a factor-3 improvement with
respect to the best value available today is obtained, with a factor-10
improvement in reach. For the $2^+$ state, no improvement is obtained due to
relatively larger error bars on the experimental phase shifts.
