The exploitation of adaptive coding and modulation (ACM) increases a very large average capacity compared to the widely used DVB-S standard for the forward link of broadband communications. The optimized physical layer configuration (PLC) will further improve the link efficiency as the actual demodulator adapt to the working decision thresholds. We analyze the link performance with different physical layer adaptations under a time variant channel of finite estimated accuracy, and propose an effective algorithm to assess the thresholds by theoretical calculations. Simulation results show that optimized parameters fulfill the target requirements with cost of minimal packet loss and system capacity due to channel estimation errors.