The sticker on the driver's door frame is the definitive value for that individual truck. It is Federally mandated and calculations are for the most part consistent across mfg's. Next best would be to look up by VIN in mfg tables.
Both values need to be adjusted for weight of absolutely everything added to the truck after it left the factory. Even required after market hitches and and receivers must be included. Overweight driver's also.
For smaller trucks and SUV's, you can often calculate towing capacity from cargo capacity or from MGVW - Dry or Unloaded weight.
First figure cargo capacity. Then subtract weight of passengers, luggage, equipment, after market add-ons, etc. The remainder is available for tongue weight.
Actual weights are what matter, but you can estimate. Tongue weight must be at least 10% of TT actual weight for safe towing. So, multiply remaining cargo weight by 10 for maximum towing capacity. 15% tongue weight will be more stable. Published tongue weights are often wrong. I have seen dry tongue weights at double the published value.
Yes, actual towing capacity can be substantially lower than sticker or advertised towing capacity. Shocking isn't it.
I wish you good luck and happy trails ahead!