You adjust your preload to adjust for certain SAG number range depends of the type of riding and your weight on whatever spring rate you have now. That will tell you if you are in the range or if the spring rate is too low or high for your weight and the type of riding.
All the hydraulics will be tuned base on feel and some visual cues. The target is to make sure all the suspension hydraulics is neither over nor under damping your suspension. The difficult part is more than often you will have to bandaid the lack of or too much spring rate, especially for OEM springs which is not custom for the rider’s weight.
One thing most people are confused about spring rate (linear spring) is the rate is constant and you simply cannot “cure” or compansate for having a wrong spring rate with the preload. Adjusting the preload does not and will not change your spring rate. There is a reason why having the “correct” spring rate is critical.
Hope this helps.