The method you're using is accurate.
You can also do it another way, just as easily-
If you use that method of raising the trailer just off the hitch, but do it with just the truck on the scale, instead of with just the trailer on the scale, then you will be weighing just the truck, and then the truck with the trailer's hitch weight on it. Subtract those 2 numbers and there you have the pin weight.
So either way will work.
Now, on the pin weight versus jack weight issue, if you were weighing just the jacks by themselves, no trailer axles on the scale, that would not be the same as the pin, because there is about an 8 foot length difference between the pin to axle measurement, versus the jack to axle measurement. The jack weight will be significantly higher than the pin weight.
