I am not entirely sure of the thrust of your question but will try to answer.

The distance between the crystal and electron trap is part of the proprietary design of the manufacturer, not something that the user has control over. (Or are you trying to design a detector?). But your second part part of the question seems to relate to something else: crystal-sample distance and working distance, these are two different concepts and do not relate to the first part of your question. You can't easily measure crystal-sample distance as what you see at the end of the detector is the collimator, not the electron trap. You can measure collimator-sample distance; if you obtain information from the manufacturer on the construction of your detector you could then know the crystal-sample distance (why do you need this?). In my experience the collimator-sample distance is kept as short as is practicable, considering the physical positions of the sample and collimator for safety reasons. (Shorter distance = greater efficiency of collection AND greater chance of collisions)

The working distance is going to be a function of your take-off angle and final pole-piece design. The optimum will be a fixed distance defined by these two factors, again you need to obtain the information from the manufacturers of your detector and SEM.