I thought the ratio was closer to 20% but found this on the web...
"Trailer sway can be a problem when towing. Maintaining proper tongue weight (~10-15% of trailer weight) goes a long way in minimizing sway problems. As you add weight to the trailer in front of the axle, the tongue weight will increase (but not at a 1:1 ratio). The closer the weight is to the tongue, the higher the weight ratio on the tongue will be. As an example, a battery or LP gas bottle mounted directly on the tongue will have almost all of its weight on the tongue (a very small amount will be carried on the trailer axle). Contents carried inside the trailer, in front of or over the axle will have most of its weight on the axle, with a small amount added to the tongue weight. Contents carried inside the trailer, behind the axle will have most of its weight on the axle, but will have the opposite effect of DECREASING the tongue weight. The axle acts as a pivot point as you load the trailer.
With that said, remember you want 10 - 15% of the trailers weight on the tongue to reduce "sway" while you are towing."
