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Peter,

Generally 3nm from a known position at the sea shore using a plastic sextant is good. More than that requires investigation. Exact height of eye, residual index error, and arc error are the most difficult to account for. Plastic frames take on a permanent warp over time leading to arc error (I have a max arc error of 5' on both of my Davis models). Expansion and contraction due to temperature changes will change index error so index error needs checking before and after an observation.  Sand in the micrometer drum isn't a good thing either when at the beach ;-)

I enjoyed our meet at Joe's Cafe :)  

Greg Rudzinski