Concatenated bit-interleaved and multilevel coded modulation with outer Reed--Solomon codes, inner Chase-algorithm-based soft-decision-decoded Bose--Ray-Chaudhuri--Hocquenghem codes, and four-level pulse amplitude modulation is considered. A semi-analytical formula is derived for estimating the decoded frame error rate (FER) at the output of the additive white Gaussian noise channel, obviating the need for time-consuming Monte Carlo simulations. The formula is used to search a large space of codes (including the KP4 code) to find those achieving good trade-offs among performance (measured by the gap to the constrained Shannon limit at $10^{-13}$ FER), complexity (measured by the number of elementary decoder operations), and latency (measured by overall block length).