Utility confusion usually begins when the source documents and the calculation live in different places. Keep the supplier bill, billing period, meter reading and allocation method connected to the same property record.
Use the method stated in the lease or otherwise agreed in writing. Common approaches include a fixed amount, metered use, equal shares or allocation by occupancy. The appropriate method depends on the agreement and local rules; software should not silently choose one.
Show the arithmetic before payment is due. Identify credits, estimated readings, standing charges and adjustments rather than carrying unexplained balances into the next month.
If a bill is corrected, retain the corrected source and explain the difference. A structured record makes review easier, but each party should still compare it with the supplier bill and the agreement.