Unfortunately when it comes to ring sizing the industry doesn't conform to one system, yes in the UK we all the use the alphabet but what I mean is how that size is interpreted. When a ring is placed on a sizing stick it might fall exactly in the middle of a size, this would be referred to as 'Center'. When a ring falls against the line of the size it is termed 'Leading Edge'. The difference in size is very minor between either but can make a small difference when it is on your hand.
The best way to ascertain a finger size is to have your finger measured by a jeweller. You could also have a ring measured that you regularly wear on that finger, one that you know is already a good fit for you. The fit should be such that the ring needs a real wiggle to come off the finger. A good time to size your finger is when you are cold as this will take into account the smallest your fingers could be at any given time of day.