True pin weight is the difference between both axles on the truck with trailer and then without the trailer.
Same is true with a pull trailer when weighing without equalizing bars engaged.
Example I checked the tongue weight of my Subaru with a utility trailer.
Car only
2180 Steer axle
2000 Drive axle
4180 total car only
Car and trailer
2060 steer axle
2380 Drive axle
2000 Trailer axle
6440 Car and trailer
Tongue weight and pin weight if 5th wheel will be calculated the same as I do not have weight distribution.
The two axels on car with trailer 2060+2380=4440
The two axels on car no trailer 2180+2000=4180
So my tongue weight is 260
Now you can calculated pin/tongue percentage.
Trailer and car weight, less car only 6440-4180=2260 true weight of trailer.
Now percentage is 260/2260=0.115 or 11.5%