I'm sorry I probably haven't explained myself correctly
I have been sent an A4 paper with those details on (see below) but it doesn't cover the 40% to 50% range so I wanted to drag the formula to include that range.
gross profit ready reckoner (inc vat at 20% in the selling price)
cost price 50% 51% 52%
£1.00 £2.40 £2.45 £2.50 selling price
£1.10 £2.54 £2.69 £2.75 selling price

regards
martin