A = the material you are mounting your movement on. This is called your Dial Thickness. This number will be the length of the threaded portion of the movement.
B = the length of the total shaft in the movement. This is called the Total Shaft Length. Please remember that if you are putting this behind glass, your shaft length need to be 1/8 inch less than the space you have between the glass and the material you are mounting the movement on. This space allows the hands to move freely.
The hole in your material you are mounting the movement on needs to be big enough to slide a 3/8 inch thick shaft rod through.