neck tension is set from your die. if you have a bushing die, you can change it by getting different size bushings. if your die is fixed, it is what it is from the manufacturer. most of us reloading precision ammo like around 1-1.5 thou of neck tension. the way to measure it is measure the neck diameter of a resized piece of brass before you put the bullet in it. then measure it again with the bullet seated. the difference between those two numbers in the neck tension.
the only two things that change seating pressure/force is neck tension and friction. if your brass is super clean (think "squeaky" clean) you can actually increase the frictional force in seating the bullet. sometimes the left over carbon residue can act as a lubricant. we actually found this out on some new brass where when the metallurgy of brass cup was changed they were coming out super clean brand new and gave a false feeling of to much neck tension. but when they were pinned we confirmed that it did indeed only have 1.5 thou of neck tension (really good for virgin brass). once we started putting the brass in a media tumbler with some flitz media polish, the seating force came way down and everything seated very consistently. and shot awesome as well