Because the reading of the window is so finnicky... one day when I had absolutely nothing better to do.... I dug out a little 2" round bubble-level I had in a desk drawer. I put the level on the bike's tanks and then found the most level place to park the bike in my garage... the oil measurement was right where it should be.
I moved the bike only about 3 feet to the right (garage floor slants right to left in most places) and I found the window level showed I was down about a quart.
You really do need to find a perfectly level spot to accurately check the oil.