Methods of controllably milling a window in a cased wellbore using a pressure differential to cause movement of a mill

A method of controlling a mill in a cased wellbore comprising:

• interconnecting a mill advancing device and the mill, wherein the mill is connected to a drill string that is located inside the cased wellbore;
• creating a seal between the outside of the drill string and the inside of a casing of the cased wellbore at the location of the mill advancing device;
• and
• creating a pressure differential between an area above the seal and an area below the seal, wherein the creation of the pressure differential causes a downward movement of the mill advancing device, the seal, and the mill.