If you know how much payload you have available for tongue weight, you can divide that number by .13 to get a ball park figure of loaded trailer weight that will put you close to max capacity.
When you go shopping for the trailer, sales people will all be taking unloaded weights, they have no clue what you will be carrying. Whatever UVW they tell you, add 1000 lbs to it, and you'll have an idea of what the loaded weight will be. You can multiply loaded weight by .13 to get a ball park of what the loaded tongue weight will be.
Leave yourself some room for possible surprise passengers and for tongue weight fluctuations. Tongue weight goes up and down during every trip.
Here's a link to a calculator that may help.