Manually enter co-product quantity in batch order

Hello, 

One of my customers has the following need.

They produce chemical components via batches. this chemical component is produced in the form of vials of 2 or 4 ml.

Each batch produces a variable quantity of 2 ml vials and 4 ml vials. The 2 ml vials and 4 ml vials are represented by 2 different items in AX.

I've created a formula with 2ml vials as item output and the 4ml vial item as a co-product (With a quantity of 1 per serie of 1)

Now the problem I face, is that if I enter a batch quantity of 100, the co-product quantity is also 100. And I don't know how to change this quantity.

I would like to be able to say that I produced 102 2 ml vials and 116 4 ml vials.

Is there a way to do this in AX2012?

Thank you

Best regards, 

Fabrice