Sight your rifle to be 2 inches high at 100yds. At 200 you will be about the same, at 300, about an inch or two low and at 400 you would have to hold at the top of the 6 inch target to hit it near the bottom.
Or, you can zero dead on at 100, then shoot at a large paper target out to as far as you intend to shoot, aiming at a target high on the paper, make int about 36 inches tall overall, fire three or four shot groups carefully, then measure the drop of each group at each range and write it down. Range to your target with a laser RF or by expedient methods (mils/MOA etc) then crank on the needed elevation for each range....
Of course, this is just the tip of the ballistic iceberg and eventually you’ll learn about jump, precession, yaw of repose and things like that. Oh, and wind effects, and relative humidity and barometric pressure and temperature...
Easy enough out to about 600 yards, on calm days.
Sub-sonic-a whole other ball game past about 100 yds.
Shooting up or down-hill adds another dimension, so to speak.
Sub sonic would be within 100 yards only. When target shooting I’m within 2” at 300 yards and on target at 400. 400 yards I can’t see well enough to be as accurate, it’s operator error. My friend can get within 2” with my rifle at that range. He’s 20 years my junior though.