For adult slippers it says, "Work even until foot measures the same as desired finished foot length from back of heel." Then you shape the toe, so the slipper ends up being longer than the actual length of your foot. For the youth slippers it says, "Work even until piece measures ½" more than the desired finished foot length. " Then you start shaping the toe. So the slipper is 1/2" longer plus the toe shaping.
You might want to compare the pattern to Fuzzy Feet. I've used it successfully for 3 people and 2 brands of yarn (Cascade 220 and Peruvian highland Wool). It says to "Take the total length of recipient's foot and multiply by 1.3 to determine the required total length of sock before felting. Then determine length of sock to knit before beginning toe decreases by subtracting 3.5 inches from total required length. "
Let's say the recipient's foot is 12" long: 12X1.3 = 15.6 = total length. 15.6-3.5"= 12.1". So this follows the knittingatknoon pattern because they say to knit the foot as long as the desired finished foot length, then to start the toe shaping.
When I made my first Fuzzy Foot, I felted it before starting the second one, but it turned out fine. I'm a big fan of felted slippers now.