I ask my customers to measure the dog's neck in the smallest part of it...tight ! ...never the old collar !!! The thickness of the collar plays an important role in sizing too . A 1ply collar , 20in from buckle to the hole has a different inside circumference than a 2ply collar measured the same way . The right size of the collar is measured when the collar is buckled up...not flat out on the table. If you have standard designs and use the same leather weight most of the time you can make yourself few collars(in one ply , 2 ply ....) from 15in all the way up to 27in and used them as guide. A custom collar should not have more then 3 holes for adjustment, 3/4in apart.
Sometime i ask for pics of the dog and its weight ...i can pretty much guess the neck size within 1 or 2in..lol...
Hope it helps