Dependency Formula for BOM qty

Hi,

I want to set formula for certain material quantities in BOM.i.e. when my customer select Number of Spindles 1056 ceratin material e.p A=1056/2=528Pieces,B=1056/4=123Pieces.

If he select Number of Spindles 1200 then Material A =1200/2=600 Pieces,B=1200/3=300Pieces

Then how to it through dependency?