IMO the best way to deal with your zero distance is to zero at 100 (which gives you the benefit of being close-enough that your zero is not affected by altitude or environmentals or much by wind, so you can get a better zero in the first place that isnt subject to future changes in location/weather), and then if you want to have a "walking around" Max Point Blank Range zero, you dial your 200-yard dope and simply walk around that way. With your 6.5cm that would have you about 1.5" high at 100 yards, and about 2" low at 250 yards if you simply aim center of crosshair at any range 250 or less. Just be careful pushing this too far, some people make it work but the built-in error stacks on top of the error in your shooting ability, and the error in your gun, and it doesnt take much to cause a miss or a wound on a deer-sized target at 300 yards if you havent practiced this a bunch.
300 yards really isnt far enough to need to change around your inputs that much due to environmentals. With a 100-yard zero and 140-ish gr lead bullet you're going to be really close to .75moa at 150, 1.75 at 200, 3.5 at 250, 4.5 at 300. At those ranges, even with a big elevation change, the difference is 1 click or less, which is less than an inch error at 300 yards--it's not going to be the reason you miss or wound an animal. Verify when you get to where you are hunting if you want to check zero hasnt shifted, but I just ran numbers for my 6.5cm and it's .3moa difference at 300 yards between where I live at about 2000' DA to 11,000'DA...which is one click on your scope. less elevation change will be less difference. If you have access to a 300 yard range you can just colllect your own dope at 25 or 50 yard increments, and make a simple dope card you can stick on your stock or scope eyepiece.
Also, you do not need a chrono if you use the truing function on your app. Just plug in box velocity to start, shoot the farthest distance you are going to shoot, and true the app to your actual drop at that distance, and it will correct your velocity for you. Then you verify the output at a couple distances to make sure you dont have an error in your inputs. This works well way past 300 yards.
All of this is prefaced by having a really perfect zero, becasue any error in your zero can be amplified throughout your corrections, so getting in the habit of making sure its perfect, not just close-enough, will be an asset as you start shooting farther. I like to fine-tune my zero off a 10-round group, not a 3 or 5-round group, for this reason.
Above all practice from positions other than the bench. That's what will make the most difference. I've been able to get prone for most shots I've taken at antelope, but there are plenty of places where grass, rocks, sage is too high and you need to shoot sitting or standing. Practice shooting off shooting sticks or hiking poles, sitting off a pack, etc.
