|Jean, Clair is not wrong, but he's not entirely right either. I have a mark V and originally I set my mast up as per Clair's description. There are 2 factors you need to take into account, first the main halyard on the Mark V does not exit the top of the mast on center, second the rail heights may not be exactly the same side to side due to the way the deck hull joint on the Mark V is done.
The best way of checking is to put an extra sail slide in the mast track above your top mainsail slid. Then attach a long tape measure to the slide and also attach the main halyard to the slide. Hoist the slide,tape measure up to the top of the mast and measure to the deck just inboard of the toe rail at the same point abeam of the mast on both sides of the boat.
If you are still experiencing a difference in performance from one tack to the other have a look at the alignment of the keel/rudder training edges the next time the boat is out of the water.
What is your hull number?
Still Knot Working MkV #75