Doesn't matter really, whatever the door says is what you should use. Tire pressure is based on vehicle weight, not the tire. Most tires can handle 60+ psi, but no one is about to crank it up that high.
I have 2 sets of wheels for my A8...17 & 18"...the label on the inside of the gas lid (like on your 2000, I assume) says 30lbs for 17" and 32lbs for 18". My last Porsche had similar recommended differences.
I would NOT assume a rule of 2 extra pounds for each extra inch of rim though. Different aspect ratio and width combinations determine how a tire "spreads" on the road under the same weight...not the rim diameter.
In racing and serious exotics circles, they often decide perfect pressure with a pyrometer (tire thermometer)...uniform temperature across the whole tread section after a calm straight road drive, without hard acceleration. Hotter in the middle = too much pressure...Cooler in the middle = too little.
So, I would try to contact someone with an S8 model that offered the 20" option (same aspect ratio/width)...or a dealer. They might even have one on the lot.
Yep, depending on your normal load too.
I've always gotten good even tire wear, goin with the Audi label's small load suggestion...front and rear the same. I rarely ever carry much more than 1 passenger and a set of golf clubs.
Don't know what they recommend for 20's and load size, though.