I have made several hats in various sizes using that pattern and think I have come up with a formula to figure out the right size.
I use m1b, m1f for my increases and (s1, ssk, psso) for my decreases.
1. Figure out your gauge for the yarn and needles you plan on using.
2. Measure your kid's head. Measure from the nape of neck, up over the ears, around mid forehead and back down to the nape.
3. Multiply your gauge by the number in inches from step 2. If your number is even, add one stitch to make it odd. (example: gauge 5.5 st/inch x 19 inch head circumference = 104.5, round up to 105 stitches.
4. 105 is the number of stitches to cast on. Now, to figure out where to do increases/decreases.
Take your cast on number and subtract 13. (105-13 = 92)
Divide that number by 6 (92/6 = 15.333333333)
This is where it is a bit tricky. The number of stitches between the first increase and decrease takes you from the back of the neck to the ear. The second number takes you from the ear to the side of the forehead. The third number takes you to the point of the forehead. Then you back track and the fourth number takes you around to the other side of the forehead, the fifth takes you to the ear, and the sixth takes you back to the nape. Six sections total (that is why you divide by 6). SO, you want your numbers from those sections to match up. If you get an even number after dividing by 6, you get lucky. If you get a .3333, you need to make one section a stitch longer on both sides, and if you get a .66666, you need to make two sections a stitch longer on both sides. I usually add the extra length to the area going from the ear to the side of the forehead. So for my measurement above I would use the numbers 15, 16, 15, 15, 16, 15 (repectively, going around). Then you can play the number of increase rounds to do by ear. If you are using a bulkier gauge, you wouldn't have to do as many rounds. I use about 8 pattern repeats before doing just the decreases. Does that make sense?!
Here are a couple more example sizes I have worked up (but you need to figure out your cast on stitches using the steps above first).
Cast on 71, 79, 85 (105, 121, 141) st. Join to knit in round.
Knit 5-7 rounds garter stitch, depending on your preference.
Rnd A: k1, m1f, k10, 11, 12 (15, 18, 21)
sl 1, ssk, psso
k10, 11, 12 (16, 18, 22)
m1b, k1, m1f, k9, 11, 12 (15, 18, 21)
sl 1, ssk, psso
k9, 11, 12 (15, 18, 21)
m1b, k1, m1f, k10, 11, 12 (16, 18, 22)
sl 1, ssk, psso
k10, 11, 12 (15, 18, 21), m1b, k1
Rnd B: Knit
Repeat Rnds A & B 6, 7, 7 (8, 9, 10) times total.
PS: Let me know if this is helpful...maybe I will post it on my blog or ravelry if it makes sense
