I've found (during my great saddle saga), that in a "normal" saddle you can get away with something that's just a 1/2cm or so wider than the center to center measurement of your seatbones and still be supported (I'm 145mm c2c and a 150-155mm is wide enough). However, if you want a Brooks, then you definitely go with the outside to outside measurement, then look for something a cm or more wider so you won't be sitting on the frame of the saddle (personal experience- I'm 160mm o2o and got the b17, and ended up having the outsides of my seatbones on the outer rivets of the saddle!)