With heels on toe-up socks, you want to return the sock to the original number of stitches. Take half of the number of original sock stitches and divide this number over the heel flap width and number of pick-up stitches down the side. I've got a diagram for a 60 stitch sock and a blank diagram you can use for your own numbers.

When knitting the little starter strip, I twist the beginning stitch of each row. After working around this strip, I use tanbark stitch for the remainder of the heel flap.