On the accuracy of the PFA: analogies between Casimir and electrostatic   forces
  We present an overview of the validity of the Proximity Force Approximation (PFA) in the calculation of Casimir forces between perfect conductors for different geometries, with particular emphasis for the configuration of a cylinder in front of a plane. In all cases we compare the exact numerical results with those of PFA, and with asymptotic expansions that include the next to leading order corrections. We also discuss the similarities and differences between the results for Casimir and electrostatic forces. 
Francisco D. Mazzitelli, Fernando C. Lombardo, and Paula I. Villar