How do you find the limits of areas on a chart?

Mxsmanic wrote:

True for slide rules in general, but they seem to have disappeared
just the same. People like new and shiny gadgets.

and they are becoming difficult to find; I have been shopping for
a decent slide rule (and a sextant by the way -- no, I am not
happy with what westmarine has in stock) for a while... you do
find stuff like that in garage sales sometimes when you are lucky
though.

--Sylvain