An important thing to remember about the preload setting on a coil fork, is that it does NOT adjust the stiffness of the fork spring. The stiffness, also known as spring rate, is the amount of compression for a given amount of extra force on the fork, and it is fixed with a coil spring.
The preload compresses the spring inside the fork, without the fork changing in length. The spring will not start compressing more until the force on the fork is more than the preload force. This means that you can use the preload to adjust how much the fork shortens if the fork is loaded only by the weight of a rider. The amount of additional fork compression by bumps or by movement of the rider is not affected by preload.
When doing a simple test of pushing on the handlebars, the fork may feel stiffer because it does not move until the preload force is exceeded. But unless the preload force is more than the weight of the rider on the fork (an undesirable situation typically), the stiffness experienced during riding will not be affected by the preload setting.
So if a fork is too stiff, or it bottoms out too easily, this cannot be adjusted with the preload, to adjust that you need to replace the spring with one that has the correct spring rate for the rider's weight and riding style. (you can also adjust how stiff it feels by modifying the damping but that is a wholly different can of worms)
The main purpose of the preload adjuster is to make sure the fork has the correct length when loaded by the rider's weight, and the geometry of the bicycle is as intended.
The amount of fork compression under the rider's weight is referred to as 'sag', and in forks with an air spring you usually adjust this with the air pressure. But air pressure actually changes the spring rate, not the preload. So any setup guide for air forks that refers to sag, should be ignored when working with coil forks.
The correct setting for the preload depends on the model of the fork and bicycle, but a good ballpark figure is for the fork to be compressed by 15% of its maximum travel when loaded by a stationary rider in a usual riding posture.
Increasing preload will put the rider in a more upright position, decreasing preload will put the rider in a more forward leaning position. Be aware that increasing preload will also limit the fork's ability to follow depressions in the surface, since it can extend less.