Howdy and welcome to the forums Zen Archer and getom59!

I keep my .223 zeroed at 100. It is capable of shooting out past 300 yards with a little practice. With some bullets, I don't have to hold very high at 300. With others, I have to hold a little high. After having gone a while without shooting it, I checked my zero in Dec. It was dead on at 25. Then a few days later, I shot at a coyote and the bullet went right over his back. I checked it again, and again it was dead on at 25. At 25, I was told it would be dead on at 250 and about an inch or two high at 100. When I shot it at 100, it was just barely hitting the top of a 10" circle target. I decided I did better zeroed at 100 and just holding over at longer ranges.

As far as adjusting your scope, you should be able to adjust the objective and the power without changing the point of impact. If your point of impact is changing when you adjust either, scrap the scope and get a better one.