Not to get too technical but it's not actually over-modulation which is a term commonly used on FM and AM but is distortion of the audio. You can't over-modulate an
SSB signal as the signal is essentially all voice audio, i.e., the signal level rises and falls with the level of your voice.
Long story short - rather than figure out how loud/soft near.far away to speak, just pick a comfortable voice and distance from the mic and adjust the mike gain control such that the audio is clear. Obviously, it helps to have someone listening...