Dan, had the same problem and found out, that the hull was not absolutely symmetric.
So if the deck was taken of the plan, but it doesn`t match exactly to the hull, you probably should blame it on the hull.
Now there are two solutions:
1. Get the hull to follow the lines of the deck by fixing it with some bolts and clamps or....
2. Mark a center line on the deck, place the hull on it and mark the hulls line with a pencil. Then cut out this line.
Hope that was out of any help.
Jörg