There is a misconception that bullets suddenly go wild after they go subsonic. The vast majority of bullets have no issue slowing down. Some may veer a little but only a few start to destabilize (you can include twist theory in there for good measure).
max distance is truly a function of your ability to adjust your aiming device to account for the enormous drop and the huge drift that can occur.
If you have a ballistic program that works out to 1000yds, you can use that data to walk you to any distance you choose. Expect accuracy to become quite generous so spotters have to be prepared to look several feet AWAY from the target.
While shooting to a mile, I had lost quite a few shots. The bullets simply didn't 'arrive'. After quite a bit of frustration, I decided to start looking well away from where the bullet was supposed to land.
At that range, a crosswind had started up and the bullet was eventually spotted a little over 2 mil dots away. let's call that 8.5 to 9min of extra windage or approx 153 inches/ 12 3/4 ft.
so no matter what you shoot, if you want to give LR shooting a try, simple do so. You will be amazed at how well the average bullet travels.
Jerry