Then how should I compare my breast size to a pit-to-pit measurement? I've seen a lot of sellers here at SF, that uses measurements right from the jacket and not around the sellers own body.
It's really confuzing when I don't have a good jacket to compare the measurements for.
Do the tailors take paid to take measurements for you?
Double the pit-2-pit to get the total chest size of the coat. This should be between 2-6" larger than your actual chest size. 2" being a very slim fit, 3-4" being a standard fit and 6" being a relaxed fit.
Or just buy a size corresponding to your chest, i.e. if your chest is 36 inches you buy a size 36 (46EU). But sometimes you'll need to size up or down from this if the coat is cut particularly slim or large, which is why the pit-2-pit is a much more useful metric.
Make sure the shoulders are the right width too, which is even more important than getting the chest size right.