It seems to me that there is no one perfect drag setting because of so many variables involved. Drag setting is not solely dependant on line test, but also size of fish, species of fish, distance from boat, cover, and other variables. An example would be throwing a large musky bait with 6/0 trebles on 100lb braid. I want absolutely no slip or give on the hook set whatsoever because driving 6/0 hooks through a musky's jaw at a distance needs all the power available and a locked down drag.
Once I see how it's hooked I don't want to horse that same fish in with a locked down drag, because a green musky thrashing with 6/0 trebles in the net or boat is pretty dangerous. I loosen the drag as this same fish nears the boat and let them make a few runs alongside the boat. Northern Pike are also notorious for not fighting super hard until they see the boat and make a suicide run as they get close. Quite different than a carp which dig towards bottom in a tug of war with predictable runs... unlike a musky or pike. Take that same musky hitting that same lure at a much closer distance to the boat during a figure eight and what I need is free spool mode on the reel and no drag except my thumb on the spool. You do not want a locked down drag when a 30lb musky hits your lure with 2ft of line out from the rod tip. You will break the rod. So you now use your thumb as the drag in free spool mode to set the hook, then allowing the fish to run once hooked by reducing thumb pressure.
Take that same lure and same rod and same line with the same musky while trolling... you no longer need the locked down drag on the hook set because the speed and momentum of the boat become an important factor in the hook set. Often going 5-7mph, the boat speed is setting the hook for you and you can afford to have a bit of slip of drag, since you could never set the hook on your own as powerfully as a 2000lb boat going 5-7 mph can. All examples of how the right amount of drag pressure is dependant on many other factors than only line test.
Just my opinion but the appropriate amount of drag is highly variable. A long rod made of fiberglass will give more than a short graphite rod with broomstick action. Likely would require different drag setting even when using same line test. I see more guys lose fish due to not recognizing when they need more drag or less drag given a set of circumstances that constantly vary.
Use a lighter than optimal drag on a musky in 95 degree heat of summer... you may land the fish... but you may also kill the fish due to exhausting it by fighting it far too long. That's an example of the right drag setting being influenced by excessive water temps.