One of the issues with plastic sextants is that they distort in the
sunlight, so keep it below decks as long as possible, and check the
index error before and after each sight.
If you want to make the sight reduction cacs easy, have a look at
WinAstro (www.winastro.co.uk)
Many years ago I did an Atlantic crossing with a taffrail log - but it
isn't too good in crowded sailing waters!
Tim