My best guess: Drop the boat in the water on a calm day and mark the waterline on the leading and trailing edge. And use these two points to take your measurment......Does this sound good to you guys?
This isn't an unheard of concept, some dinghy's set their mast rake by floating the boat in a pool and measuring where the main halyard lands in the boat.
my issue would be, when your taking your measurement, would you have the crew aboard the boat? where the crew sits determines the attitude of the boat and may dramatically alter your readings.
My history includes sailing maricats which are fully cantered and banana shaped hulls. there is nothing straight about them! The way we dealt with them was to lift the rudder to the vertical position then sight the bows.
I am thinking it may be useful to sight the rudders with the centerboards...
it is something I'm going to look at with my boat shortly.