It made me crazy having to figure out how to adjust the pattern to fit my heels. Every time I would knit a pair of socks, I'd have to do it all over again. It would be just a little bit different than the last time. There should be a formula somewhere to make the calculations easier. After a fruitless search, I decided it would take less time if I did it myself....

So, here's my no-fail formula for constructing a slip-stitch heel on toe-up socks...

So far, you've knit the foot, and done the gusset increases. You've also completed the short rows for the shaping on the bottom of the sole. I usually do magic loop, so I'm only working with needle 1 (instep stitches) and needle #2 (sole + gusset stitches), and this discussion will be discussed from that point of view. If you use DPNs, you would have the instep stitches on needles 1 & 2, and the sole/gusset stitches on needles 3 & 4. You will have a certain number of stitches on needle 2, and will need to reduce these stitches down to the number on needle #1 when you beging the leg portion of the sock.

So, the given values you'll need for the formula are:

- The number of stitches initially on needle #2, call this A
- the number of stitches you want to end up with, call this number B

Row 1: Knit C stitches, ssk, turn.

Row 2: Slip 1, Purl D stitches, P2tog, turn.

Row 3: (Sl1, K1) E times, ssk, turn.

Repeat Rows 2 & 3 until all stitches have been worked.

You need to calculate the values of C, D and E, using the values of A and B which you know:

C = [(A+B)/2] - 1

D = B - 2

E = (B-1)/2

Let me give you an example. We can look at the Slip Stitch Heel Basic Sock from Wendy Johnson's book Socks From the Toe Up. After the gusset increases and the short rows to set up turning the heel, you will have 63 stitches on needle #2. So A = 63. You need to reduce this number down to 33 stitches. So B = 33. We can calculate the other values we need for the heel flap using the formulas above.

C= [(63 + 33)/2] -1 = [96/2] -1 = 48-1 = 47

D = 33 - 2 = 31

E = (33 - 1)/2 = 32/2 = 16

Plug these values back into the instructions, and you'll get:

Row 1: Knit 47 stitches, ssk, turn.

Row 2: Slip 1, Purl 31 stitches, P2tog, turn.

Row 3: (Sl1, K1) 16 times, ssk, turn.

And that's exactly how Wendy Johnson wrote the instructions in her book!