The test Phill and I would like the measurements from is taken on land. Just measure the distance from bowtip to bowtip. Then hoist the mainsail and sheet in hard (could we come up with an easy and repeatable way to sheet in with a known force?). After you have sheeted in, say enough to flatten the sail as good as possible, take the measure again. This would give an indication on how much flex there is in the hulls under load.