Depending on what the smith asked, and the customer asked for the throat length may be setup for long or short bullets... longer is typically the safe side.
So the BTO is something you the customer can ask the smith to setup for you so the base of the bullet is just above the neck should junction.
However with most hunting bullets - they are to long anyway so you have to seat the below that...
But it’s a good exercise to look at - don’t forget you have a magazine OAL issue to worry about as well.
