If the biggest thing that you shoot at it will be .338 LM or Edge or a variant of those two, then 3/8" AR 500 plate is really all you need at that distance. You might get away with 1/4, but I would default to the bigger plate to eliminate any risk of damaging the plate.
Personally I have 1 - plate that is 3/8", and it gets used at and above 1,000 yards with my 338 LM Improved, and my friend's 338 Edge.
I have a bunch of 1/2" plates and they get used anywhere from 200 yards out to 2,300+.
When I get my 2 mile gun (375/416 Barrett), the 1/2" plate will be used exclusively, and no closer then 1,000 yards. (Basically going to treat it like its a 50 BMG for energy)
I'm lucky and my home range has 3/4" AR500 plate targets permanently mounted, so I can shoot closer than 1,000 yards.
JeffVN