For the problem of boxed printed circuits embedded within a stratified media, efficient formulations based on the Method of Moments have been suggested which involve the computation of modal series appearing in the representation of Green's functions in this context. This paper suggests an efficient truncation criterion in computing these modal series which guarantees attaining an acceptable accuracy in final results while avoiding unnecessary computational effort. Different structures are examined in the framework of printed circuits packaged in rectangular shields to verify the validity of the suggested criterion using rectangular and triangular mesh schemes.