Most cards have an interest free period, so the best way is to pay it off in full before the due date.
The big mistake people make is that they don't pay it off in full. They have an amount due of €503 and they pay off €500. The full interest is charged on each item from the date of purchase.
The other big mistake people make is that they pay a few days late. Again, they are charged from the date of the purchase up to the date it is paid off. With a mortgage or other loan, if you a day late, you pay 1 day's interest. With a credit card, you could be paying 57 days' interest for being a day late.
The limit is irrelevant.
If you have not paid off your bill by the due date, the best strategy is to pay it off as quickly as possible.
Skrooge's explanation was excellent.
Think of it as a series of separate transactions. If you buy something today for €100, you will be charged €100@15%/365 each day, until you pay it off.
Brendan