I used to obsess about my oil level in this bike and the sight glass used to drive me nuts. Now I just drain all the oil out that I can (2014 with front and back drain), clean the screen, soak a new oil filter, pop it in and button her up and add two quarts.
My understanding is that these motors are not sensitive to minor oil differences and if it is low enough to be an issue it would not be in the sight glass and the rockers would clack like crazy. If you overfill, you'll be spending $21.95 plus shipping on a new air filter...so I've been sticking with a "two quarts and that's plenty" sort of a approach.
Most times on a level surface my oil level is at the top mark in the top 3/4 of the sight glass. I found a level spot in my shop and marked it with a sharpie - traced where the center stand is when level so I can reproduce the control. After a decent run I set it on the center stand let it idle on that level spot for one minute on my watch and shut it off. I then take off my gear and give it a minute and look - usually it's right on, sometimes I need a minute more and its fine.
I do however, change the oil at 2000 miles maximum and look for any sign that it's loosing oil.