Counts, control totals and a sample walked end to end — DEFINED BEFORE THE LOAD rather than argued after it.
The timing is the whole lesson. A reconciliation agreed in advance is a test the migration either passes or fails. A reconciliation constructed afterwards is a negotiation, conducted by people who already have a number they want to be right.
COUNTS. Records in the legacy extract, records in Fusion, and the difference explained record by record. "Close enough" is not an outcome; every missing record has a name and a reason, or the migration is not reconciled.
CONTROL TOTALS. A monetary sum, on a stated basis, from both sides. State the basis in advance — gross or net, which currency, as at which date — because most reconciliation disputes are two people summing different things and both being right.
A SAMPLE WALKED END TO END. Three to five records, chosen before the load, followed from the legacy source through the file, the interface tables, and into the application, field by field. The sample is what catches the failure the totals cannot see: a value that landed in the wrong field, a date that shifted, a default nobody chose.
THE EVIDENCE A FINANCE OWNER ACCEPTS is a signed page, dated, naming the extract, the counts, the totals and the walked records. Write the template before the load, get it agreed, and fill it in as you go — because reconstructing it later depends on an extract that may no longer exist.