I finally got down to measuring the luff curve of an old BG sail today and measured it to have approximately 9mm luff curve with max luff curve from 600mm from the top of the mast to 1000mm from the top. The curve was even throughout the luff of the sail. Why do I want to know this? Because I need to match the mast bend to the luff curve of the sail.
So with my new sails and spars, I set up with my mid point settings and started pulling on the backstay to see how I could achieve a similar mast bend to the luff curve. I ran a piece of elastic from the top of the mast to the bottom of the gooseneck to give me a reference point and was able to match the luff curve exactly. There is a max limit of about 8mm mast bend but instead of the sail breaking down at a hard point, it just flattens evenly.
The picture shows the mast with the blue elastic reference line. There is 10mm of mast bend, which is the limit of the sail luff curve and with no Cunningham applied the sail is perfect, much flatter at the top with enough power lower down to drive the boat. This would be a perfect top end a rig condition on flat water. Proof of the pudding will be on the water.
Note the sail looks a bit funny at the bottom but when I applied the cunningham the sail was perfectly smooth and balanced. I will be testing this out at the first opportunity.