You said shaft, so measure the shaft. It can be different from the ferrule especially if somebody has been abusing the shaft with abrasives. So I generally check it in several places.
When measuring for a tip I just measure the end of the ferrule. I usually take three measurements to allow for brain farts, human error, and the ever present gremlins......the old measure twice cut once idea....but I measure three times..... But of course the easiest is to just put the tip up to the ferrule and see if it's big enough. :wink:
If it's for specs for selling etc, I measure in at least three places on the ferrule.