300 WM doesn't even have a full caliber neck to start with, so that's not something you can realistically do all the time to start with. Full-caliber is too much, not a good rule of thumb.
I use half a caliber (like you mentioned ) as a guide for short bullets where there's no feasible way to ever get to the lands - example of 58gn Barnes Varminator in my 243, at 0.070" longer than the book spec I'd only have a half-caliber of bullet in the neck so I stopped there even though I still had distance left before I hit the lands. I think the bullet stayed in the modified case by the time it touched but it was a comically small amount of contact so I worked from book COL longer since there was no risk of accidentally jamming a bullet. Book COL was 80% of caliber in the neck, and there was no way to get to 100% in the neck because the start of the bearing surface would be below the case mouth before the bullet went past the end of the shoulder.
Long bullets gets into how much of the case neck is even sized by the die, I can't recall a bullet/case combo where the bullet didn't seat pretty much entirely in contact with however much of the neck I was sizing, which again from my recall is always at least half a caliber.
But the real answer to the question is another question - what's the seating depth to where the bullet you want to use would contact the lands? Start there, work back, check to make sure there's enough bullet in the neck to keep it in place but other than that don't sweat it and work the seating depth tests.
