Sometimes bullets are not uniformly round and so the general practice is to take an average of the diameter.
I think otherwise if the measurement is pretty close to whats in the book, use your judgement to select the bullet closest to it. That said their are myriad variations of three ringers and Enfields and so the ones in the books only represent a few of them. As Jim would say if he'd log in more frequently, "its just a three ringer..." To some extent the same can be said for many of the Enfield types.
