One thing that I saw when first shooting subsonic at long range was that I always had the time to re-acquire my sight picture after pulling the trigger. So if the bullet landed somewhere that made it visible (like dirt) it was a simple matter to adjust the holdover then hit the target on the follow-up shots. This is how I am able to consistently hit a target at 500 yards with my 300 whisper in my AR-15, or other subsonic carbines.
I have them zeroed at 200 yards normally. I have to know my range very well and am not able to guess it very well either. This is where a ranger finder comes in handy. Past 200 yards, the subsonic bullet is headed downhill rather fast. Using the 458 cal 500 gain bullet above, from 200 to 250 yards it drops 21 inches; from 400 to 410 yards it drops 10 inches. The good thing is that you can expect very consistent velocities from a good subsonic load, especially one with a small case capacity. The whisper loads I shoot typically have a standard deviation of less than 15 fps.
Ranb