I have never shot subsonic ammunition. Hopefully someone with experience in this area can help. However, to answer the question it really comes down to what it is the min and maximum distances you want to shoot and how big is the target?
For me shooting a 22-250 at around 3800 ft/s, I zero my thermal at 1" high at 100 yards. This most likely will be very different for a 300 Blackout shooting subsonic ammunition.
The average coyote night shooting distance is going to vary greatly depending on part of the United States, terrain, and the hunter. I shoot most of coyotes around 125-175 yards. I also get lots of 200-400 yard shots on multiples after shooting the first coyote.
If talking to coyote hunters in wooded terrain, I would have to guess most of their shots are under 100 yards.