Co/by-product

Hi,

I have item A.

After production, I have item B1, C1, D1 that have standard cost.

I want to calculation cost for item A following formula Cost of item A = Total Cost of product - Cost of Item B1 - Cost of Item C1 - Cost of Item D1.

Who can help me to set up co/by-product?

I want to fix amount when use TCA. But It only have percent.