Use separate accounts and keep a dated ledger tracing each transfer from its lawful source to a business expenditure, supported by statements, contracts and receipts. Show original source and the full path, not just a closing balance. Keep the household reserve in its own account so the enterprise's funding can be read without untangling groceries.
A ledger that survives a stranger reading it
Each line should answer three questions: where did this money lawfully come from, how did it travel, and what did the business buy with it. Sale proceeds, accumulated employment income, a documented gift or a loan each need their own supporting file, and a loan's collateral should be identified because security over personal assets is treated differently from security over the enterprise. Gather statements while they are still easy to request.
Ledgers fail on the oldest entry, not the newest. The recent transfers are documented in detail and the origin is summarized as savings, which is the one point a reviewer cannot check. Work backwards from the business expenditure to the earliest identifiable lawful source, and stop only where a document exists.
Where a bank has purged statements beyond its retention period, request what remains now rather than after the account closes, and record the request and the reply. A gap that is documented as a gap is a different problem from a gap that appears as an unexplained deposit.