OldmanFSCA is echoing the principles worked out for maximum consistency and reliability by NOIH (Naval Ordnance at Indian Head) in the early '80s. They found for the standard small arms primers that they all needed to be "reconsolidated" for best performance. "Reconsolidation" means the primer and anvil are "consolidated" into an assembly by the primer maker, and seating pushes the anvil in deeper, thus reconsolidating it.
Allan Jones refers to this as "setting the bridge", which is apparently an industry term. I don't know the standard NOIH used to measure the effect other than the standard H test, but I have seen a description of the primer test apparatus McDonnell-Douglas used in assessing primers as including UV and flame temperature and explosive impulse measurements, so it may have been something fancy llike that.
The bottom line was that while Olin and Remington had recommended reconsolidation of 0.002"-0.006" for all small arms primers up until 1981 (IIRC), NOIH recommendations were narrower and called for 0.002" to 0.004" of reconsolidation with both sizes. The only thing I've seen different from that is specific to Federal primers, which we all know use a slightly different form of lead styphnate in its standard primers. In Precision Shooting magazine in the mid-'90s Federal was quoted as having recommended 0.002" of reconsolidation for small rifle primers and 0.003" for large rifle primers.
The only commercial tool I am aware of that lets you control reconsolidation directly is the K&M primer gauge tool, but it is slow to use as it requires both a measuring step to allow for the depth of the individual primer pocket and the height of the individual primer going into it, and then the actual seating step. It is slow going, and I vaguely recall one of the benchrest shooters writing around then (Dick Wright, maybe) mentioning that he had given that up because of the tedium involved, but that his wife had continued the practice and seemed to be beating him at all the matches they'd recently shot in.
The first person I am aware of to bring the idea reconsolidating could have a significant effect to the public was the late Creighton Audette, whose last three articles for PS Magazine were about the "analog" nature of primers. By that he meant how much they were reconsolidated affected how hot and consistently they performed. Resulting velocity SD was one metric used to validate this. Up until then, many benchrest shooters favored seating primers only until you could just feel the feet of the anvil kissing the bottom of the primer pocket. Audette's experiments showed you need more pressure than that, and he proposed a primer seating tool design that later was realized by Sinclair as their hand priming tool. It was to give better feel to primer seating, though the later-developed K&M tool I mentioned lets you read consolidation directly on a dial indicator so you don't have to guess by the feel.
However, putting special tools aside for a moment, it seems just knowing you need some degree of reconsolidation puts you ahead on the subject. In the 1995 Precision Shooting Reloading Guide, Dan Hackett wrote:
"There is some debate about how deeply primers should be seated. I don’t pretend to have all the answers about this, but I have experimented with seating primers to different depths and seeing what happens on the chronograph and target paper, and so far I’ve obtained my best results seating them hard, pushing them in past the point where the anvil can be felt hitting the bottom of the pocket. Doing this, I can almost always get velocity standard deviations of less than 10 feet per second, even with magnum cartridges and long-bodied standards on the ’06 case, and I haven’t been able to accomplish that seating primers to lesser depths."
Dan Hackett
Precision Shooting Reloading Guide, Precision Shooting Inc., Pub. (R.I.P.), Manchester, CT, 1995, p. 271.
So that gives you some idea about what you are looking for when you experiment with priming. Whether getting your velocity SD down from 20 fps to 10 fps is something that matters in and of itself depends on the range you shoot at, but keep in mind that here a low SD is being used to indicate consistent ignition. At 100 yards, velocity SD doesn't contribute much vertical stringing, but getting barrel time and pressure waves consistent still may shrink your groups. Unfortunately, you have to try it to find out.
As to how far below the back end of the case you should seat a primer, Hounddawg is correct that there is enough variation in primer pocket depth and anvil height after initial consolidation to cause that indicator to be too imprecise to be relied on to produce the most exact possible effects.