Hi, I don’t know the norsea 26 . If you intend to race the boat under a portsmouth rating , the only reason to need a number your local club fleet or administrator should start with an estimated number by comparing to similiar boat. After that a number can be established by submitting numbers to us sailing . But it will take along time as ussailing has not updated their numbers for years. They have basically abandonanded the rating system .
It is still well used in the U.K. But mainly for their local boats.
The rating system works best for small boats that are raced regularly.
Hooped that helps. Warren