Originally Posted by socalfamous87
much of a difference does this make?
Well that depends. It depends on the die, the press and the shell holder.
As I stated earlier, this is the "Generic" way to set up an FL die, a more precise way is to use a gauge like the Hornady/Stoney Point Headspace Gauge.
When a case is run through the sizing die it makes contact with the case walls, this sizes the case body, as it does so the case begins to grow longer which in turn pushes the shoulder forward. The FL die needs to be adjusted deep enough to contact the shoulder of the case and bump it back.
Depending on the press the 1/4 turn past shell holder contact removes the slop in the linkage of the press resulting in much more consistent sizing. It's called "camming over".