What percentage of the combined weight of a vessel and trailer should be used to calculate a trailers tongue weight?

If the trailer is properly balanced, the tongue weight should be 10% of the combined total weight.

Most boat trailers are not properly balanced. You have to check the balance. If you know the total weight, you can lay the trailer tongue onto a bathroom scale. You will likely find you need to shift the contents of the boat to get close to 10%. There are alternate techniques for boats too heavy for the bathroom scale (boats / trailers over 3,000 pounds).