Again, it depends on the size of your target. I use a 4x32 balliatic plex reticule for medium game on a 6.5x55. Works great from 0 to 300 yards, as that is what I have it set up for. Works well on coyotes also. Having said that, I will use a 3.5 PU for further.
Freedom is not free. The best of us always leave too soon.
Doesn't so much depend on the power of the scope as to what you expect to do with it. Keep in mind the guys at Camp Perry compete at 600 Yards wiht the 5.56 with iron sights. If you know the dope on your ammuniton and of course the wind issue. Would not be hard to hit at 600. But if you are talking about varmint size targets I too would have to agree 400 yds. would be close to the max I would even consider. In fact you should possibly be thinking a variable. Also getting a Sout Rail or forward offset mount for mounting the regular sized scope on. Otherwise you can have eye relief problems.