What size of groups are you getting at 100 yards right now? Take that number and times it by 2 for 200 yards, 3 for 300, etc etc.

So if your rifle is shooting 1.25" groups at 100 it "should" be capable of 3.75" groups at 300 yards, 5" groups at 400, and 6.25" groups at 500.

Also, if you plan on using your current 30-06 and extending its lets, I would upgrade that scope to a VX-3 4.5-14 or something similiar.