These charts are from Judy Gibson’s "Putting Me On Socks,” which she acknowledges as originally contributed by Peggy Pignato. Chart 2 appears as originally published. Chart 1 has been modified from the original, but the calculations follow Pignato’s original proportions.
IMPORTANT! For Chart 2, you need to subtract this number from the total length of your foot in order to get Measurement #3. (Example for a 9" foot circumference: 9 - 4 = 5. Therefore, I cast on at the toe and knit for 5 inches before beginning the gusset.) This length does vary by the person, though. So if the socks are for you, try them on to make sure they're long enough before starting your gusset. And if they're not for you and you have the possibility of taking an accurate Measurement #3 from the intended's foot, do so and use that measurement instead!