Take a piece of brass thats already been fired in your gun, pinch the neck a little bit out of round with a pair of pliers & then start a bullet into the case with your seating die just enough to grip the bullet, now insert the long seated round into your chamber & let the rifling seat it for you. All you have to do now is measure the over all length (OAL) & subtract .010" or whatever amount you want the bullet to jump.
Be careful seating the long round or the rifling will grab the bullet from the case, you'll have to tinker a bit to see how much to pinch the neck with the pliers, you can measure it faster than it takes to read this.
Dick