Thought it was just the case of setting sag to 25-30mm then if your bottoming the forks you may be able to increase the compressions dampening but you most likely have a inadequate spring, don't see what geo has to do with sag, geo is designed for the forks and shock to be set at the correct sag and that's it.
If your running less than 25mm sag on the front and bottoming the forks you need a heaver spring.
100% the compression damping is not there to stop bottoming out.
Compression *damping* controls the speed of the of the fork dropping through the stroke to the end of the stroke. Caused by bumps, braking and cornering forces.
Rebound *damping* controls the speed of the of the fork rising back up the length of the fork. Caused by throttle and spring tension.
Neither of these are a means to STOP the fork moving...if you have got that point you've hydraulically locked the damper and that's very bad. You may think you're helping it with compression but really you're just ruining the ability to handle bumps react to the changes in the road.
Damping is about keeping the tyre in contact with the road. It's generally only adjusted, once it's right, when the oil degrades (though changing the oil would be better).
Preload, you correctly point out it's a range ~30-35% of travel in the front and around 25% in the rear.
You use it to set sag and set your BASELINE. So for me that's 6mm (6 turns).
Preload is not set and forget. It depends on the usage. If the preloader can turn 12 times then you'd ideally want to be be set between 4 and 8 turns out of 12. (33-66% of the range) any more OR less you would absolutely require new springs.
