I think your wise to design the gantry so that the spindle is centered over the the base. When you think about the weight of the gantry top and the forces of accel and decel going back and forth with the moment being 12-20" away from your contact points is going to make the top want to swing with each direction change. Looks like you have a good start going. I will be starting a build log soon and that is a definete consideration in my design. Even using bearing blocks and precision rail the weight of the gantry at that moment is a factor and I really don't want it offset far from the centerline.