Haven't built one (yet) but as I understand it, it's supposed to be right around the center of the trans tunnel arch. Keep in mind also that stripped down there isn't a lot of weight so the CG isn't terribly critical. I thing this may be one of those situations where close enough is good enough. The pictures I have looked at vary a little up or down and nobody seems to have a problem one way or the other. Also, as you add things back on it will change minimally. Just my 2 cents worth.