I think your summary captures the essentials of the design. However, as I indicated in my post, there is a difference between seeing lines (visibility) and resolving lines near to each other (visual acuity). A line much narrower than an arc minute should still be visible, but the two arc minutes would give a desirable safety factor. In the example that you give the size of the umbra (or penumbra) is the limiting factor.
I would think that features of a dial should be several times that indicated by theoretical visual limits. A sundial should be a pleasure to view -- not an eye test. JMO.
Gordon Gordon Uber [EMAIL PROTECTED] San Diego, California USA Webmaster: Clocks and Time: http://www.ubr.com/clocks
