While making the periscope reticule and the AOBF finder wheel I spent a lot of time to make sure they're within 50 m of a correct range, even at 4000m up. I never used the map to check up my settings, I used the Mission Editor instead. During rough seas however you can be off by 200m no matter how good you are. So I can safely say that it's as good as it can be, the rest depends on your ability to estimate the miliradians and to interpret the AOBF data.
Accurate range is only important if you want to calculate speed through plotting. I never do that. If you can place yourself ahead of the target you have enough time to close in on a good angle, stop the sub completely and do a fixed wire speed calculation. If you're closing in on a FAST ship, you can also use the 45 mark bearing for that, and just add YOUR speed to the speed the AOBF gives you. Just before shooting, I usually get another range reading because I like to see when the torpedoes hit, on the stopwatch. As long as you keep the gyros lower than 30 degrees, you can mess up with the range by as much as 500m. Actually, I've never seen a torpedo hit exactly on the chronometer mark. They usually hit where I wanted them to do, but 10-20 seconds later. Maybe there's something wrong with how the game calculates range/time.
As for shooting at 4000m plus, that's a crap shoot. Unlike in SH3, the ships in SH4 actually get slowed by hitting the waves. As far as ship handling and convoy scattering is concerned, Lurker has managed to get a better result than you can find in SH5
So don't get used to shooting at ranges greater than 2000m.