I just took delivery of my Kevlar Blade here in OC, CA. When I have it fully assembled, I'll take boat and trailer to a scale at a local metal yard and weigh them together and then the trailer by itself. That should get it pretty close.
John, Blade ? (haven't unrolled the sail yet)
Not sure that will be accurate enough. Best check what the tolerances are on them; or are they "proper" hanging hanging scales. I assume by you saying that you will weigh boat+trailer and then trailer that they are "big weight" drive on ones for cars / trucks / lorries etc, If so I don't think they are going to be anywhere near accurate enough to pick up +- 5 or 10kg
Still will be interesting mind.