Chances are pretty good that you already have all 3. Most of us outdoors geeks do. (Well, maybe not a Ranger - but a mirror compass anyway.)

Try taking the measurements yourself on a topo and a known spot.

I have this as an exercise for students in a course using my grab-bag of compasses and find that getting angles to 5 degrees is about all you can get from first-timers and not using a mirror. I've not really spent the time to see if I could do better with lots of practice.

My point for the exercise was to have them plot their bearings and get a triangle rather than a pinpoint to demonstrate that there is ALWAYS uncertainty in measurement.