Xmax and Xmech

If I want to design a ported or a 6th order bandpass box for a speaker, the Xmax is exceeded. If I design a 6th order bandpass where the Xmax is not exceeded, the box is not buildable (rear chamber is tuned too high and the port have unreal dimensions, or front chamber is tuned too high - no low bass). I know that the Xmax of the speaker means that this is the max linear excursion where is no distortion, the Xmech means the maximum mechanical excursion which if is exceeded, the voicecoil hits the magnet (bottoming out). In normal I just cannot find the Xmech parameter. My question is that there exists a a rule or a formula (or something) what helps me to determine the Xmech parameter of a speaker, or Xmech is individual for all speakers?