Hi Alexandru,
Each type of designed system has its own requirements (defined by recommendations of ITU-T, IEEE, etc.) for BER and, therefore, Q-factor. It is known that these two parameters are related BER=1/2*erfc(Q/sqrt(2)). According to ITU-T recommendations, BER is limited by 10^-12. The Q-factor is usually in the oreder of 10^1 power. Threshold parameter is used to define your decision criteria. By doing that, you can maximize your BER and corresponding Q-factor.