This section is from the book "The Manufacture Of Boots And Shoes: Being A Modern Treatise Of All The Processes Of Making And Manufacturing Footgear", by F. Y. Golding. Also available from Amazon: The Manufacture Of Boots And Shoes.
There are two measurements that are usually termed heel-measures - one the distance round the foot as usually taken, and the other the" long-heel, a measure especially useful in lasts. The latter is taken from the top of the instep, from the same point used to take the instep-girth, and round the corner of the seat of the last-Both of these measures, the heel-girth and the long-heel, may be approximately calculated from the table given Let a = instep measure; 6 = long-heel measure ; r = rise; c = heel-girth.
Then a + 1/3a + 1/2r = c adults' a + 1/3a - r = c children's a + 1/2a - r = b children's a -1/2a = b adults'.