The Lyman and Sierra loading manuals I have say to seat the primer .003 to .004 below flush.
I'll be darned, your right, I found it in my Lyman book. It's actually .003-.005 with a nominal of .004,,
Like I say I guess I've always just gone flush, never gave it a second thought, and ain't had trouble yet,
(knock on wood)
, I wonder if it's another rule like, "never exceed max loads"??
Don't know about the Redding, but my Lyman pocket uniforming tools are adjustable. Never messed with'm, I just left'm at the factory setting