Your pattern calls for a heel flap and I've never done heel flaps, I always knit toe-up and usually do short row heels, worked on half the stitches. I think others here have done heel flaps and will better understand what you want to know and be able to help you. Meanwhile, I think I'll look at knitting heel flaps and try to see what you want to know. I really like the socks you linked to.
I should never overestimate my basic arithmetic skills.