How do find out the collar measurement?

Learn Apparel Merchandising. 
If you have no idea that, what will be the collar length of your Polo shirt. Then pls follow the below instruction.

At first you need the following information :

i) Neck width of the polo shirt. (Suppose it is 22 cm)  
ii) F/neck drop. (Suppose it is 4 cm)

Rule:
= (((Neck width + Neck drop ) X 2) -3)-12.5%

= (((22+4)X2)-3)-12.5%

= ((26X2)-3)-12.5%

= ((52)-3)-12.5%

= (49)-12.5%

= 42.9 cm