I hesitated to write this note, because I don't want this to turn into a "how to check your oil" thread. That being said, I do want to point out, while many people do check their oil exactly how you have described above, I would hesitate to say that is the way to properly check the oil.
The only way which BMW has outlined to check the oil on a boxer is to put the motorcycle on the center stand, wait five minutes, and then view the oil through the site glass.
Using the method that hopz has described above is said to be sure that the oil is out of the oil cooler, etc prior to checking it. However, considering that oil is in the oil cooler when it is running, I'll leave it up to you to decide which method is better.
Lastly, I will note that the oil quantity sensor warning will show up earlier on the RTs that I have worked with when the oil is checked using the side stand for five minutes, then center stand for another five minutes, vs using the BMW suggested method of center stand right away, and then checking the site glass five minutes later. Using that method, my experience has been the site glass and oil quantity sensor agree.