I want to verify my turret at 400 and 600 yards. I know the high power targets are round but would a square or diamond shape be more accurate for centering the reticle at distance? What do you prefer?

Round or square is all I've used, pretty easy to quarter them with the reticle.

Reticle size, distance, target size, scope power, and mirage all play a role finding accuracy.

I use a 17 in. square plate panted white, with a 2 inch black dot for an aiming point.... Works great.
I also set up a camera with a35x zoom to record my shots...review at the range or at home. I record my load, rifle, conditions etc. Also very neet to show any visitors the video, which will include the bullet trace. Spooks people when you tell them they can see the bullet!

I like a cross, easy to keep centered with cross hairs.

I make a 1 moa circle drawn on cardboard with black marker (1" at 100 yds, 5" at 500, etc). I make the outside border thickness proportionate to the size of the circle. This has proven to be a pretty nice target picture for me.

This is about like reloading. Depends on your unit. Over the years I've developed about 20 different targets I can generate with my PC and printer. Some are great with my high power (20X and a quarter inch bull's eye) scopes and others with the 2.5X(about a 2.5 inch bull's eye. Still others work best with open or peep sites. Regardless of design, one should be able to see the bull's eye such that he can generate precise and repeatable placement of his sights. One can do moa shooting even with irons if the target meshes well with his sights. Again, it simply depends on the specifics of your equipment.

Square steel and diamond shapes on paper.

this..............

Fixt....ha ha....

More specifically, I like paper plates of various sizes when not doinkin' square steel.....

This AR-500 plate is 32"x34" and the group is ten shots from 900 yards with one of those sorry assed Remington 700 rifles!

308 Win/Remington M700 factory barrel/action by Sharps Man, on Flickr

dang good shooting there