I'm still learning about this too, and I'm finding that there are a lot of factors to consider. You mentioned curb weight but every pound that has been added after the factory can take away (in my case) ten pounds on the towing limit. Even 15lbs of floor mats in my instance mean 150lbs less I can tow.
You also didn't mention tongue weight or percentage of tongue weight to trailer weight. And there are some recommended safety buffers. And the type of hitch you use matters.
So there's lots to consider. I found this calculator helpful. Be prepared to be shocked how much less you can actually safely tow. For example, my SUV can pull 8,900lb but I'm looking at 4,000-5,000lb trailers.
http://changingears.com/rv-sec-calc-trailer-weight-tt.shtml