What is the minimum amount a bullet should be seated into the casing?

The factories and the older ballisticians of days gone by, believe that one caliber depth is required. Practically, any depth where the bullet will remain concentric, particularly important for rounds in a magazine. Half of a caliber isn't unusual.

Thanks for the reply Bob.
I had also heard the one caliber depth theory, but can't get 69gr. Match Kings within a .100" of the lands in my AR15 using it. I wasn't sure if it was an old rule of thumb or set in stone, so I thought I would ask, thanks again.