The best preload setting is the one where the majority of times your backside stays put. That is, if you hit bumps or dips, your suspension will absorb one or the other and keep you and your wheels firmly in contact with the surface they are sitting on.
It takes trial and error with adjustments of preloading both the springs down or up until you feel confident you've found the best setting possible depending on the shocks you have.
Btw, if you're happy with the front it'll make getting the rears sorted that much easier as I believe both work together to achieve optimum performance.
Just remember, this can sometimes take time to sort out but Keep in mind the importance of keeping your butt as firmly in your seat as possible which will mean your shocks are adjusted properly and doing their job.
Staffo
