John,
Here goes what Soler says about the size of numbers in a sundial:
The greatest power of human sight to recognize numbers varies
largely depending on external conditions like illumination,
elevation and so on, but we can take as a reference value
something between 3 and 7 minutes of arc.
This gives the '1 cm -> 5 m' rule: a number of size H can be
recognized from a distance of 500*H at most.
This agrees more or less with what other people have said.
but from my point of view (did you get it? :-) maybe the
5~rule is too optimistic.
Regards,
Anselmo
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