I think of it more like a cache, not a config value. So maybe we should use XDG_CACHE_DIR. Or, better: I write a small tool "xwiicalibrate" which, when run, shows the MP calibration offsets that a user should write into xorg.conf.d/*.conf. This would provide a quite user-friendly way to calibrate it, and, at the same time, avoid any separate cache/config file.