There are too many variables for a 'one size fits all' approach.
Rider weight, luggage, pillion, on/off road, condition of shocks etc.
Your best bet is to try various settings in your normal riding conditions and choose that which feels most stable and comfortable.
I have mine on the setting which allows me to have as much foot in contact with the floor as possible and it's been that way
for the last 50K only adjusting for two up or fully loaded configurations. Handling has always been adequate.
The best starting point is set everything back to how it is in manual.
When considering pre load changes it’s about weight of rider and passenger plus luggage.
Bmw have set the standard settings for their average rider weight which maybe 13-14 stone. If you are significantly above or below that weight you need to add (fatty)or remove preload (skinny). Or when you’re loaded with pillion etc.
On the rear shock there’s rebound damping which again is best set with the manual settings. Rule of thumb is increase the damping with preload increases and vice versa but only a click at a time. Too much will make the rear slow to return after a bump and feel harsh. Too little and it’s bouncy.
Front preload is by stepped adjuster and ideally leave it as per manual unless you have significantly added or removed rear preload.
The principle we are looking for is that the suspension is balanced front to rear and the geometry is correct as per how the factory produced the bike.
This ensures the optimal handling characteristics.
The BMW telelever and paralever Suspension does mask the effect of changes a bit but when it’s in the sweet spot you should know it’s right!
Alternatively go to experts like Revs Racing and they’ll give you a nice cuppa & a quick set up.