Yes, you should always mention the model, year, and often milage when you ask a technical question. Also, your location sometimes makes a difference.
Since you have a 2008 with low miles, we can perhaps give you a place to start. Your weight and your passenger's weight are an important part of the equation, too.
If you are of "average Goldwing weight", probably 220-240, I would try about 10-12 solo. If your co-rider is also "average" (180-ish, say) probably around 18-22. Add luggage, crank it up a little more.
Your suspension preload is designed to keep your suspension kind of in the middle of it's travel so it will give damping both in compression and rebound without bottoming out. Goldwing suspension is less than stellar, to say the least. As your supension ages, it gets less and less effective. You kind of need to set this by feel, mostly. If you are bottoming out a lot (your passenger gives a big "ooof!" as you hit a good bump, or starts demanding that you announce the bumps) then you probably need more preload. If you're cranked all the way, then it may be new shock time. New shock time usually comes at around 25K miles or so.
Also, Honda routinely doesn't fully fill the actuator, so it doesn't start working until it hits 4 to 8. Put your bike on the centerstand, and run the preload to zero. Be vewy vewy quiet, as Elmer says, and start running the preload up. When the pitch changes from "EEEEEEEE" to "OOOOOOO" stop. Read the number, that is where you preload stops pumping air and starts pumping fluid.
So, the number also depends on where your adjustor actually starts working.
A man, much wiser than most, Tom Finch, who owns TailWind Trailers and is a long time high mileage Wing rider posted an explanation, with illustrations of the rear suspension on the Wing.
His conclusion was to raise the setting to the max 25 and leave it there.
Whatever setting one picks, it does not change how the shock operates. It only changes the preload which affects the ride height some and keeps the suspension from bottoming.
I am assuming one needs to make sure the pump is full first.
When I was still 2 wheeled, I did as Tom suggested, without telling my wife.
On the next ride, she asked what I had done to the rear suspension as the "bumps" and "dips" were not nearly as harsh as before.
I never changed it from 25 after that!
OK, so everybody has a setting that works for them, based upon their weight and riding style. At 143 lb solo, the stock spring might actually be the correct weight for Goldwingfd. I think it's about a .85 kg spring. I suggest a 1.1 to 1.2 for most riders, based on weight, passengers, luggage, riding style, etc. The stock spring us usually way to light for the "average" GW rider.
That said, the only way to know what spring is really right, and to set the preload correctly is to measure the sag.
The Wing has 4.1" of rear travel. You want measure where the suspension sits when unloaded, then measure where it is with typical load, rider, feet up, to see where it sits. (you usually need help to hold the bike up and measure). You are looking for a loaded sag measurement of about 25-30% of your 4.1" for travel, or maybe 1 to 1.4 inches of sag.
This will give you as much compression travel as possible while leaving some for rebound.
This is the only way I know to actually set the number correctly. It takes the riders weight, the actual preload setting, and any luggage / saddlebag clutter into account.
If you are near Ormond Beach in Florida, PM me and we can measure it up for you. If you're not near by, it's a touring bike, c'mon down.
Otherwise, crank in a number, see if you like how it feels.