Either the boattail or the pressure ring of a flat base should be just up into the neck past the shoulder neck junction.
You will find that different bullets have different lengths of bearing surface and how far forward the come to meet the lands. If it erodes a little, you can change to a longer bullet and still be in the lands.
having said that, for single shot loaded bullets, in 1k BR, it is not uncommon for as little as .200 of bullet be to in the neck.
BH
BH