I was wondering if any other toe-up sock makers would find this useful when they're making a charity sock without a specific size in mind. After casting on a toe and making the increases, you sometimes wonder how long the sock should really be, especially if you're using yarn scraps or changing the yarn in a pattern. I compared about 30 friend's and family's measurements and found the following is true for just about anyone over 4 years old.
After finishing the toe increases, knit an inch or two further, then stop and measure across the width of the sock (not circumference). Multiply the width by 2.25 and you have the approximate length. Then subtract how much the heel will take up (half the width) and you'll know when to stop knitting the foot and start the heel.
If width is 3 inches, x 2.25 = total length of 6.75. Subtract 1.5 (heel), so start the heel at 5.25 inches.
This is easy and works well, and I don't like math!