The mass balances required by most product safety standads are there essentially to prove that your traceability systems work but there can be spin off benefits tp process improvements when you analyse wastages.
In broad terms a mass balance is a percentage based upon the quantity of a raw material received divided by the quantity of that material used to make good product plus the quantity of the material wasted.
In real life it is never that straight forward, but for the aluminium it isn't too difficult.
In the example below, assume that one coil is used to make three different finished products:
Used on Finished Product
Number of Finished Product cans made
Waste from Punch Press
Other Process Waste eg trim
Mass Balance= __________________ X
(aW1+aP1+aO1) + (bW2+bP2+bO2) + (cW3+cP3+cO3) x100
Most can makers will have carried out detailed process capability studies and providing that the can size/design is the same for the three products, unless there is a breakdown on the line, W1,W2 and W3 will be the same value and similarly P1,P2, P3 and O1,O2,O3.
The example assumes that the start of Finished Product 1 coincides with the coil change, which is unlikely, so you will have to understand your own process lines to find the start and end points.
The broad principles are the same for the other raw materials, but I have to say that it will be a nightmare to calculate mass balances on inks. Irrespective of any individual process considerations, the potential error in scaling up from minute ink weights per can to the millions of cans made will make a percentage reconciliation meaningless in my opinion.