I would look at the GVWR of the trailer and assume 10-15% of that weight. That way you're prepared if you're fully loaded.
Tongue weight has a lot to do with how you pack your trailer. If you put EVERYTHING up front you may be at more like 25% tongue weight. If you put it all in the back, you'd be closer to like 5% tongue weight (I'm guessing at the actual percentage, but just giving you an idea of how it works).
The brochures are rarely accurate. I know my dry weight was lower on the sticker of my trailer than what the brochure showed. You can easily take the brochure tongue weight and add close to 300lbs for batteries, propane, hitch system and whatever you load in to the front of the trailer to give yourself an idea. Best bet is going with GVWR and assuming 15% to give yourself a peak number to stay under.