Luis and Carl, every one is right, but to keep it simple if the geometrical centre of each sail is taken (for convenience sake this will be accurate enough for the purpose in hand) by scaleing a drawing of the boat and the sail plan, mast etc, and then bisecting the angles at the head, the tack, and the clew, then by projecting a line in towards the centre of each sail, where they bisect will be the approximate centre of effort of each sail, not only the fore and aft but also vertically, if these points are joined between the different sails and a point is taken scaled relative to the percentage difference in area of the different sails, we will then have the combined centre of effort. Project that point down at right angles to the hull and it will then give the fore and aft C of E at the hull.
For the centre of lateral resistence, if the boat has centre boards, then assume that the point of maximum thickness of the board is the centre of lateral resistace. If it is something like a Hobie 16 we found years ago that, although the actual centre of lateral resistance of the Hobie will vary quite dramatically depending on the attitude of the hull in the water (ie if the bows were depressed then the CLR would move forward and if the bow was raised in the water the CLR would move back), by sitting the hulls on the ground without any mast rigging fittings rudders or rudder cases, then the point at which the hulls balanced was as near as damit to the CLR, (not very scientific, but it seemed to work OK). When it comes to measuring the point of CLR on the hulls, it doesn't matter if you measure from the bow or the stern, it is still the centre of lateral resitance! I don't quite understand why you ask that question????