The thing about long range accuracy is not that the caliber itself is limited to 100 yards, but that the sights & trajectory on a typical HUNTING gun limit it to 100-150 yards or so.
Yes, you can hit at 1000 yards, but those types of competitions & shooting are not done with a Guide Gun. Longer-barreled rifles with specialized sights.
There's just too much of a rainbow flight travel, it's far from being a flat shooter.
If you want to zero your Guide Gun for 200, you'll need to learn where it'll impact at more traditional closer distances.
If you want to zero at 100, that's more practical, and you'll still need to learn the gun farther out if you want to shoot farther out, using the same sight setting.
Bullet drop in the .45-70 at 1000 yards is measured in several feet.
In the .45-70 world there's two camps: Hunting/Defensive vs Long Range Target. (Yes, there are those who like to play a little with intermediate distances.)
You don't use the same guns interchangeably.
Denis