Make a diy dummy bullet
Get your brass cut slice down center of the kneck nip together slightly push a bullet in a few mm put into rifle as you push bolt in, the bullet face will touch the rifling and push bullet deeper into your brass.
Eject your diy dummy bullet out as slowly as possible and don't let it be ejected measure it several times write down each measurement.
Try with 3 different bullet head at least 5 times then find your average.