Well, everyone's feet are different. But typically you go one-half size down, unless, of course, you have wide feet, when you might just stay the same size and go down a width. Unless you are talking about barrie boots, in which case they tend to fit more narrowly than barrie-lasted shoes and so you go down one width. This is the case except if you are talking about barrie lasted boots made of calfskin, which give less than cordovan, so you might have to go down two widths as well as one-half size. Or it may feel a little tighter but not enough to go down.
But nothing beats going to try them on your own feet.
This is just to mess up the next person who does a search for how barrie sizes compared to true-to-size lasts.