Hi Nell,
indeed, you can calculate the mass balance somewhat from both a single process (exporting inputs and outputs to excel e.g.) and also from the calculated inventory (again, copying and pasting to excel), but the balance will often not be perfect (e.g., due to omission of incineration air input, or cooling water input, or also since quite some products are not specified in mass but in pieces or volumes e.g.).
Theoretically, an LCA system has to be balanced, since you do not have any storage terms in an LCA model.
Best wishes,
Andreas