Measure the circumference of her foot around the ball of her foot. Multiply this # by your gauge. I usually decrease then by about 5% for snugness, but if you want them to be worn for a while you may not want to do this.
I then round up or down to the closest # divisible by 4 because I do a 2x2 rib on the cuff. If you are doing a 1x1 rib, round up or down to the nearest # divisible by 2
Is this what you meant?!