In the specific case I have told you, a PSK would have two signal levels, corresponding to two states with a phase difference of 180 degrees. Therefore, the signal value is either above zero (for ‘1’, for example) and below zero (for ‘0’). So the threshold should be set at zero, a value in between. I choose the values in accordance with the values of symbols created by the PSK encoder, which I observe using M-ary visualizer. I think it is OK if you do not use the exact value as them, but still I use a value a little more than them.