I think practically, that adjusting the air cap (by changing the oil level) is not really all that critical for our "plush" front suspensions. But I found another series of posts from about 4 years ago by Tom Cutter who suggests that it is something to be considered. He's big into suspensions (he races and sells a specific brand of suspensions and springs that can be customized) so I figure he knows what he's talking about. Tom made reference to this chart by Wibers:
http://www.docstoc.com/docs/107690689/Gabelfedern-_Fork-springs_-WILBERS-PROMOTO
One section is for BMW and the one column to the right that has a cloud over it is the air cap over the oil. Probably a lot more going on in this chart, but it does appear that the amount of oil could be fine tuned for each rider. But in general, the value listed in Haynes or the manuals will cover a wide range of riders and situations.
http://www.docstoc.com/docs/107690689/Gabelfedern-_Fork-springs_-WILBERS-PROMOTO
One section is for BMW and the one column to the right that has a cloud over it is the air cap over the oil. Probably a lot more going on in this chart, but it does appear that the amount of oil could be fine tuned for each rider. But in general, the value listed in Haynes or the manuals will cover a wide range of riders and situations.