If I'm reading the pattern and what you've done correctly, you work to the middle of the piece, turn, and work to the outer edge. Your slip sts should be done on the outer edge, not in the middle where you split for the heel. HTH and that your stocking turns out to your satisfaction. It does look like a fun knit. If you'd like to post a photo of your work, we'd love to see. A WIP photo might help clarify things.
I should never overestimate my basic arithmetic skills.