Preload is used mainly to set the ride height. While adding more preload will make it seem stiffer on the very lightest of hits, that is only because it will take more energy to get the spring to start compressing. When the fork spring does start compressing, it will compress at the same rate as always. That factor can not be changed by adding preload. Basically, you will make it worse on very light hits and the same on large hits.
Also, there is only so much space for the springs to compress. If you were to add a lot of preload, the springs would still try to compress 11.whatever inches on a huge hit and they would clang as the coils bound from lack of travel, leading to fork damage.
Basically, to get your forks set up properly, you are going to need the correct spring rate, including the correct amount of preload.
In addition, modern suspension theory is more of a stiffly sprung / little preload / lightly damped combo instead of the older "lightly sprung / highly preloaded / stiffly damped" style. For example, a 2007 KX250 has a .44 fork spring rate. A 1997 model has .40, and a 1987 model has around .37.
Without further information, I'd suggest a .42 -ish rate spring for your weight.
I hope that helps explain it.