As downtheroad said it's kind of personal. I'd take my present trailer's weight and subtract it's dry weight to get a rough number of how much stuff we pack. That would give you an idea of how much you will probably carry in the new trailer.
Example: I know my trailer weights around 12,000# with a dry weight of 10,500# so we added 1,500# of stuff and mods (ie 2nd AC unit). If I was looking for a new unit, I would know to add 1,500 to 2,000# of stuff (to be on the safe side) to the dry weight. I would then take 25% for my new pin weight. My present pin weight is 22% so I have a margin of error built in.
Enjoy shopping.