You've probably heard of the three most important things in business; location, location, location. We in reloading I think the three most important things to get right are; case prep, case prep, case prep. Skip a step and your SD's fly out of the window.
Yes the correct amount of powder is important but if you haven't resized your case correctly throwing the correct amount of powder means nothing. If you haven't bumped your shoulders back you will effect case volume, therefore pressure, therefore velocity of your bullet, no good if you are shooting past 600 yards. If you haven't cleaned your primer pocket will you get a consistent ignition? If you haven't got consistent neck tension the pressure build up will vary and so will velocity and so will point of impact. If your cases are stretched will they chamber? If they are split will you blow your gun up?
Case prep, case prep, case prep, it will make you more consistent beyond 600 yards. I believe it is why I am able to shoot consistently out beyond 1000 yards. However, if you think this is all a step too far you should probably stick to shooting 100 yards at your local gun club or you may get frustrated!