Please advise how Sage calculates the P & L Incomes

Sage calculates the incomes from the typical "4000 Nominal codes" - The sales codes. In the default Sage setup they normally range from 4000-4007. These figures are then pulled through to the P&L once revenue is recorded via the Sales Ledger accounts for the chosen period in question. The same figures will be used on the VAT Return (VAT 100), should the business be VAT registered.

Hope this helps.