If you have to set the preload so high to get the proper static (loaded) sag that it leaves you no free (unloaded) sag, that indicates the spring rate is too low for your weight. You're giving it too much preload to compensate for the spring being too soft. A spring of the correct rate will let you set the preload in a way that the static and free sag are both acceptable.
http://web.archive.org/web/201804260...ension_Preload is a good writeup explaining how sag works with the spring rate. In short, sag is just setting the starting point for spring action, so as to optimize the operating range of the spring. A spring of a certain rate will always compress down to a certain length under a given force, regardless of how much or little preload you have on it. That's simple physics.