Most of the mass of a buildup is in its lower portion. Because of this, it is the 5000 foot winds aloft and the 10000 foot winds aloft charts that need to be studied. These two wind directions generally will not be that far apart from each other. The storms will follow this wind direction. Armed with this knowledge you can make sound, safe decisions.
Most dinghies actually measure the distance from the mast tip to some fixed point aft.
In the Hampton this point is the centerline where the deck meets the transom. It is important to have firm rig tension, and pull the ram on enough to keep the mast straight. This requires sighting up the aft edge of the mast to confirm that it is straight. Just a slight bend will change your measurement significantly. If you use the larger wire in your jib luff as your primary head stay, you will need to have your jib up for this measurement.