This paper presents results of an extensive parametric study, based on Electromagnetic simulation, for square loop type frequency selective surfaces (FSS). Consideration was given to the physical attributes of the FSS, such as the loop dimensions and spacing, substrate thickness and dielectric properties, for several frequencies and plane wave incident angles. Correlation analysis and evaluation of input parameters on the FSS performance were performed, yielding to a refinement of the corresponding commonly used equivalent circuit (EC) model. The latter, allows for better estimates of the frequency response of such (canonical) FSS structures. A correction factor for the classical formulation is proposed, in which prediction errors are significantly reduced from 18% to 6% in presented cases. A 6GHz in-house fabricated FSS prototype is also measured in an anechoic chamber, with results being in good agreement to the simulations.