Would this be for the second hand car margin thing?
That seems to still be the way to do it.
350 * 20/120 gives you the VAT content of the gross profit. On the invoice you need to enter the nett profit. So from your figures £350 is your gross and £58.33 is your VAT. You need to either check the ‘enter line totals inclusive of VAT’ and enter the gross in the line total box or enter the nett value in the box as you usually would.