The example you gave is inconsistent with what you said the boss wanted. If the cake cost you 0.30 to make, then a retail price of "cost plus 70%" would be 0.30 x 1.70 = 0.51. What you did in the example was "cost plus 70 pence," which is an entirely different thing. So before we can help you with this you need to tell us precisley what it is that your boss asked you to do. By the way, often when talking about price mark-ups we talk about "margin," which is calculated as price minus cost divided by price. It turns out that multiplying cost by 3.33 would give a 70% margin. So since the boss said that was wrong it seems he is not asking for 70% profit margins.