Ten percent less than the front usually works OK, so if you are happy with the front that is the way to go.
Most new shocks are set at 16/18 mm preload and you are instructed to adjust it + - 3mm at the max, or change the spring.
My local guy doesnt like any more than 18 mm preload, anytime.
When you nail it both ends feel better even though you are only adjusting one.
For a road bike 50mm F / 45 R works sag works well for some, but it is up to you, you can set it up soft or hard or anywhere in between, as long as the balance, front to rear is maintained and you have decent damping.
And tires.