I personally do not care to shoot at steel closer then 200 yards but that is me personally. THat has as much to do with that I am shooting them with and how hard they are on steel targets as much as anything else.
TO be honest, if your shooting a conventional cup jacketed bullet, anything past 100 yards should be plenty safe. With the harder accubonds I would not shoot closer then 200 yards because there will be larger bullet frags coming off those bullets when they hit the steel. They will also be harder on mild steep plate as well at the closer ranges.
One thing to remember is that if you can get a bit of cant on the steel it will direct the bullet frags down into the ground. Since you have the chains welded to the back of the plate, I suspect you already have this angle when the steel is hanging.
With my Allen Magnums I do not shoot at steel closer then 500 yards, rips the gong right off the chains. Need bigger chains I guess!!!!
Kirby Allen(50)