Humm, I thought the 90-degree should be used for squaring, but I might be thinking wrong about what you are doing.
Identify the angle needed with the button (mark), run the angle along top or bottom of strip and cut! If you’re still unsure about it there is a good video on “Simply Quilts”. Key in quilt ruler in search, it should be the second show listed “Ask Alex”:
http://www.hgtv.com/hgtv/pac_ctnt_988/text/0,,HGTV_22056_58466,00.html