To be able to calculate net when entering Sales and Purchase Invoices

Currently in Sage 200 when entering an Invoice directly onto the SL or PL, you have to enter the Goods and VAT amount and the Gross is then calculated. I believe there is an option in Sage 50 for &Calculate Net,& so they can enter the Gross amount and the Net and VAT is calculated. It would be good to have this in Sage 200 as the Gross box is greyed out, so some customers have to work out the Net and VAT manually.