The sign NUMBER was added to the classifier F in order to follow the general ASL rule of identifying the noun before using that classifier. It is important to use the facial expression of "oo" to describe the size of the written degree symbol. The movement was also added to the sign to incorporate how the symbol is written
A whole pizza measures 360 degrees. When it is sliced in half, each half measures 180 degrees. If you can't eat half the pizza, you might want to slice it in half again. Each slice now measures 90 degrees.