NavList:

An interesting question for homemade kamal-like devices: where should the string go? How should it be held? The traditional description (almost all based on that article from Calcultta 1836) tells the observer to hold the string with the teeth. For low altitudes, less than 20° or so, that works fine because the distance from the teeth to the angle-measuring tablet is then nearly the same as the distance from eyes to the tablet.. But a modern user would do better to bring the string to the bridge of the nose. This eliminates a possibly significant "instrumental parallax" when looking at angles high in the sky. :)

Bob Peterson, you wrote:
"However, I find that calibrating your hand is also 'handy' "

Yes. For linear measurements, I have used the length of my index finger for most of my adult life. It's reliably four inches or 10 cm near enough for almost any use. I do that when I don't care much about the exact number. For "improved" linear and especially angular measure, there are so many "found objects" with reliable sizes and angular dimensions at fixed distances. Spacing between lines on common (US) paper (and index cards) is almost always a quarter of an inch. At 14.3 inches distance from the eyes, those are 1° wide (14.3 inches is 57.3× a quarter of an inch). But how much can you trust that angle? That's worth some thought... A quarter of a degrees. Yes usually... Nearest tenth of a degree? Probably... with care. Certainly not to the nearest minute of arc... and seconds of arc? No! And how much can you trust the angular size of your finger? It depends on a great many choices...

The weird wikipedia article on the kamal suggests that an "issabah" width is 1°36'25". Seconds of arc?? At arm's length a second of arc is about the size of an E.coli bacterium. Clearly that precision is nonsensical. Of course the traditional angular unit does not have to be taken "literally" either. A "finger" doesn't have to be an actual finger. We can consider the name "finger/issabah" as origin story or etymology (and a "zam" is an eighth of that). Historians of navigation usually count it that way --not literally a finger. Even so, the evidence is all over the map, and the practical angular size of the "issabah" historically might be between 1.5° and 1.9°. The "finger rule" is a "rule of thumb". ;)

Frank Reed