Can be calculated in days, or hours even. 5.6 x days worked per week. Or 5.6 x hours worked per week. That would be the legal minimum. They can then offer anything above that. This assumes you are in the UK.
So your 16.8 is right. So you need to ask your employer why they are saying it's 12 days - maybe it's 12 days plus x number of days bank holiday but do those bank holidays fall on your working day? Bank Holidays 2013/14/15 - many will be on a Monday, Good Friday is always a Friday, and some vary the day of the week. Sit with a calendar and work out what bank holidays fall on your working days over the next year or so, then check with your employer that you get those off in addition to the 12 days holiday... plus check that the total for the holiday year (holiday year runs from the date specified in your contract/written statement, or if not specified it runs from the day you start work) is 16.8 or above.
Sorry that was unclear. The 3/5 of the 28 includes bank holidays. You don't get bank holidays on top of that. So you'll be paid for 16.8 (17) days holiday each year in total including the bank holidays.
Most years there are 8 bank holidays so this sounds like the reason the numbers don't match, full time workers probably have 20 days they can choose to take leave and 8 bank holidays making the 28 days total. Depending on the days you work it might still work out right for you but if not then have a chat with HR.