Money math - pounds, shillings, pence

Hi there. I need some help here, because I'm in the process of fixing some subtitles I found in Portuguese language. It involves the monetary system with pounds, shillings and pence. About halfway the movie, Albert is doing math about how much it will cost to court Helen Dorse, taking into account all the chocolate boxes he might need to buy during a year. So, his math is 28 shillings a month leading to 16 pounds, 6 shillings after one year. But if I'm not mistaken, it would be 16 pounds and 16 shillings, since 1 pound equals to 20 shillings. 28 (shillings per month) times 12 (months in a year) equals to 336 shillings, so 16 and 16. Am I correct and Ms. Close had her line wrong, or is my math wrong?