This is the most precise.
Primer / Gauge Assembly | Primer Pocket Tool | Reloading | Seat the primer until you "feel" it just touch the bottom of the primer pocket
kmshooting.com
What you want to experiment with is variations in primer
crush. This is the only tool I know of that shows you the exact crush of each primer and pocket.
Primer cup heights vary. I routinely see .004" variance in cup height. So seating primers to a certain depth is pointless unless you have measured all your primer cup heights and then sorted them accordingly. That also assumes uniformed pockets.
Knowing this, why do so many BR competitors use the 21st Century or Sinclair Primer seaters? Because there is very little to be gained by tuning with primer crush.
Using a good quality primer tool, seating until you feel the primer bottom out then giving just a tad more gives you 90% of what you need. The 21st Century and Sinclair give you that. Even a CPS, IF you buy the longer handle, has good enough feel to seat with a little crush.
The K&M Primer Gauge allows you to see exactly how much crush you are imparting with a dial indicator.
When I tested the K&M Primer Gauge vs seating primers to a certain depth, I improved ES by 5 FPS in a well tuned LRBR rifle chambered in 6 BRA. That was it--5 FPS in a case that uses 30ish grains of powder, in a full-on BR rifle.
That was meaningful in that rifle, because having a 5 FPS wider velocity node can help keep the gun in tune better during a match.
However, I can think of no other application outside of BR--Long Range or Short Range, where a 5 FPS wider node is useful. Even then, the conditions have to be very good for it to show up on paper.