1. convergence = ch. long x constant of the cone
rearrange to give:
constant of the cone = convergence / ch. long.
2. Convergence is correct at the parallel of origin. Constant of the cone is sine (parallel of origin), therefore parallel of origin is inverse sine (constant of the cone). Use the 'inv' or '2ndF' buttong on your calculator and the 'sin' button for inverse sine.
3. With damping as far as I can find out.
4. Indicates (falsely) a turn toward the nearer pole, i.e. toward the north or a right turn. To indicate a right turn when not turning the compass card must turn left, anti-clockwise (as viewed from above).