I think it comes down to what you expect the ACK to provide. And to be clear I am commenting on "Field ACK" and not "Ship ACK":
Should it kill or heavily damage everything that makes a single pass no matter the speed or maneuver?
Should it just "remind" you not to get to close and that you shouldn't try that again without a plan?
Have no "real" effect unless you try to land on the field?
Personally I have never thought that the ACK should be strong enough to "protect" a single fighter making slow circles in it while trying to bait any enemy fighters to come down into it. But likewise you shouldn't be able to land at the enemy field and jump out and grab some coffee and donuts from the O'Club either.
My opinion is that the "Field" ACK setting should be low enough so that if you try to make your 3rd pass in through it your probably going to take some damage. This allows fighters to attack the field with their guns but also requires them to take out the ACK positions if they plan to stay and implace a "low cap". In other words a "good nuisance" piece at the fields.
Good posts from many others on this and their observations. I know what I experienced at A27 during Frame 01 with an ACK setting of .5 and thought that to high given our mission and AC and attack profile. I think the .4 setting is about right given what I expect the ACK effects to be.