Steering by compass can be tiring. Your heading varies as your boat reacts to passing waves and swells, and you must perform continuous mental averaging to keep your boat on course.
To lessen fatigue, you can steer toward a solid point of reference on or above the horizon along your intended heading. This allows you to check the compass periodically rather than continuously.
When you can’t find a suitable landmark that lines up with your course, you can use one that’s offset to the left or right.
To do so, you’ll need to chart or approximate the angular distance from your heading to the landmark and use this angle to align the landmark with your compass. Just remember that the angle widens as you approach, so you’ll need to update your approximation from time to time.
You can use your hands to make quick angle measurements. A single finger held up at arm’s length marks about 2 degrees, two fingers mark about 4 degrees and so on. A closed fist covers about 12 degrees, and a closed fist with an extended thumb marks about 15 degrees. –Bob Sweet
Condensed from The Weekend Navigator, Chapter 11: Eye of the Mariner