Some bowyers measure a longbow's length going straight from NTN, while others (such as myself) measure the bow's length following the limb's profile. As a result, there is variation in the industry. I believe one should follow the profile, as AMO does. That, IMO, makes the most sense as that is also how a recurve's length is measured. The only reason I can see to go straight is because it is easier (and it would allow one to advertise a shorter bow than it actually is...and short bows are kind of a fad now).