It' depends on what you are really doing with it. I sight mine in dead on at 100 yards. There is little chance of having to shoot further than that where I hunt so there isn't any point in sighting in high. I used to do that because I read it was the right thing to do, but quikly found the flaws in that practice. No point in sighting in for an inch or two high at 100 yards when most shots will be closer to 50 or less.
Now if you were mainly hunting fields where shots can range anywhere from 50 to 500 yards I would sight differently. I was always told to sight in to where I could hold on target at the inteded ranges I planned on shooting.
If you do plan on shooting far make sure you gun can accuratly shoot that far. I won't even touch on shooting deer at long ranges with a 243. The internet police would be all over it.
try to get it at least bore sighted after mounting the scope with a laser or optical boresight. If you can't do that, start at 25 yards and see where your hitting and adjust accordingly and then proceed to go out to what ever distance your looking to sight it to. Use a good sled or sand bag rest.
Easy way to bore sight is put the gun on bags without bolt in.Look in bore and put target in center of bore.Without moving gun,look in scope.Move scope OPPOSITE till crosshairs are on bull and bull is in center of bore.Then put it dead on at 100yds.
Standard calibers I like to zero for 200, making 300 yard shots easy without calculations. I start by taking the scope and turn both dials through the full range of adjustment counting the clicks, adjust them back half the counts to center the adjustment range.
Then go to the method 410er mentioned. At 25 yd with gun solid in position, bolt removed, perfectly center the bullseye in the bore. Look at how it looks through the scope, if it is off more than just a little in L-R and the scope base is adjustable, center the scope L-R using the base adjustments. After this or if there is no base adjust, adjust the crosshairs to about 1/2" high on the bull. This will put bullet impact about 1/2" low at 25 yd when aiming at center. With most scope setups and flattish standard calibers this will put you around 1.5" high at 100, real close to zero at 200, and on hair at 300.
I fine tune then for zero at preferably 200, if not then high at 100 using a ballistics table for the correct amount required for the 200 zero.
Really depends on what you're shooting. Minute differences at 25 turn into big differences past 100.
In my 243 with a 90 ballistic tip, 1.5" high scope, if I zeroed at 25 it would put me over 3 high at 100 and 4 high at 200. I'd come back to zero at 300. This is a similar trajectory that I set up for my 257wby and 300 ultra.
I just shot my 270 2 days ago. Right on at 50 gave me a little over an inch high at 100. So sighting in for right on at 100 makes my shots at 50 around an inch low. No big deal on deer size game. It's rare that I shoot more than 100 yards at game.
If I left it at an inch high at 100 yards no harm would be done. Take that for what it's worth. I like to hunt with a rifle that hunts to point of aim at fairly close range. I may have to thread the needle though some brush or something. I would rather do that at close range than be right on at longer rangers. I wouldn't make those shots at longer ranges. I would simply rather aim a little high.
It sounds like you are just getting started in hunting. So I Will assume you will be hunting in the woods. So a 100 yard shot would be a long shot for you.
Depending on how fast your bullet is you should sight your rifle anywhere from dead on to 1/2" low at 25 yards. The faster your bullet, the lower you want to hit at 25 yards.
Shoot your rifle first at 25 yards. First adjust your cross hairs exactly as explained by TundraGriz above. He is dead on right with that method. Then shoot one round at 25 yards. Adjust your 243 so that it is hitting just low at 25 yards(anywhere from dead on to 1/4" low). Then go to 100 yards. Sight your rifle dead on at 100 yards if you are a woods hunter. And sight it 1-1/2" high if you are a field hunter.
Once you can cut the X at 100 yards then walk away from the bench and don't go back.
Take the rifle for a walk in the woods where you can safely fire your rifle. As your are walking look ahead and pick out targets. It can be anything. A stump, a log, an ironwood tree, a large mushroom. It does not matter what. Just practice shooting in the woods. Shoot at anything, just be safe. Practice at the bench is OK, but it is not real world practice. Learn to pick your shot at a target in the woods, then do whatever you need to make that first shot count, and hit your target. Tom.