material quantity calculation column not appearing

Hi,

I am working on material quantity calculation. requirement is as follows

Product A has to components B,C

If assay is 100% then B is 10 KG and C is 5 KG will be input for producing 15 KG of A.

If assay of B is 95 then B should be 10.52 KG and C known as filler material should be 5-0.52 i.e. 4.48 KG

Now I have created a characteristic as ASSAY and assigned to a batch class and assigned this class to the above three materials

Now in the material quantity calculation screen in master recipe of Product A this Assay column is not appearing to write the formulas. what settings I need to do so that ASSAY coumn appears in the screen .

Thanks

Vinamrath