Mine tapers out a bit as well, it measures 9 1/2" at the top, tapering out to 12 1/2" at the bottom. I cut it 35" long and cut the hemline again at the end if need be.
If I am feeling fat I cut it a bit larger. If I feel skinny I might cut it tighter with a big seam allowance. If I feel vampy I don't flare it as much, then the skirt ends up tighter and is a "sexier" cut. Oh my Lord did I just say that? LOL You know what I mean right?
Moral of the story is---don't FRET this! Draw some straight lines and go. If you're worried, I wear anything from a size 4 to 8 American/ 10-12 UK/ 38-40 Euro. Depending on your size go up or down a few inches, it won't be a big deal, trust me. Email me if you're freaking out, I'll help you.
Knit is such a great fabric to work with because it's forgiving when you wear it. I made my "pattern" by drawing straight lines on freezer paper. I use this guide over and over again.