Define the commercial measure
Agree which preparation, transport, warranty and selling costs belong in the measure you use internally. Keep the definition consistent. Otherwise a comparison between vehicles or months may reflect a change in costing rather than a change in performance.
Keep it separate from the VAT calculation
Commercial profit and the VAT margin scheme are not interchangeable measures. Scheme eligibility and the records required depend on the transaction. Confirm the correct treatment with your adviser rather than working it out from an internal profit column.
Match costs to the vehicle
Use a reliable stock reference so invoices can be connected to the right unit. List costs expected but not yet invoiced when reviewing a sale. Make the estimate visible and update it when the actual amount arrives.
Follow settlement through
Part-exchanges, refunds and other settlement differences need to be reflected correctly in the records. Reconcile the sales information to the accounts and bank, and investigate exceptions while the transaction is still familiar.
Look at time as well as margin
A vehicle can show an acceptable sale margin while holding cash for longer than expected. Review stock age and funding commitments alongside the profit measure. That gives the buying conversation more context.
Where to start
Write down the definition of each margin measure used in the business. If the workshop, sales team and accounts team use different definitions, resolve that before building another dashboard.
Further reading: HMRC: Circumstances affecting the second-hand vehicle VAT margin scheme.