About the margin of error in a direction
It's impossible to eliminate errors completely. Therefore, let's calculate in advance.
No matter how accurate the map is or how precisely you draw the lines, errors will inevitably occur. This is because the Earth is round.
On the spherical Earth, maps represent Japan as a flat plane. Various Japanese maps are typically drawn using the Lambert conformal conic projection, which prioritizes area accuracy and may introduce some direction errors. As the distance increases, the errors become more significant.
Especially for distances exceeding 200 kilometers, errors become pretty substantial. Whenever possible, aim to be in the central area of the region!
It's recommended to avoid using the area within 5 degrees on either side of the direction line for Houi-Tori. While accurately drawing the direction line is essential, the more critical aspect is selecting a destination where errors are calculated in advance!
Points to note
This "error area" is derived from the maps I've tested and judged to have the slightest error. Please be aware that the errors could become even more significant with maps other than the recommended ones.
Bonus
The national map of Japan is centered around Tokyo. Therefore, the directions observed from the Kanto region are relatively accurate. However, there can be significant errors when measuring directions from areas west of Nagoya or north of Kanto. The errors become more significant as the reference point for direction measurement moves westward. For measuring directions from Kyushu to northern Kanto, setting up an area with a generous margin of error is necessary, so please be cautious.