The famous 1/10 of a mile is the precision that a full size metal sextant can theoreticaly be read to. Also Pub 229 the official tables for ship navigation can be worked to. That said it is impractical for real use and almost impossible from a sail boat. Most people I know use Pub 249 (Air Navigation) which is a nearest mile table and really only read the sextant to the nearest mile. This is an acknowledgment of the environment we are working in. It should also be noted that 249 (http://www.landfallnavigation.com/gsightreduc249.html) covers the entire globe in three volumes, while 229 requires six (http://www.landfallnavigation.com/gsightreduc229.html). Where the metal sextant pays off is the stability of the instrument from changes due to grip pressure and temperature. The added weight also makes it more stable when taking sights. The approximately three pounds isn't too heavy to hold, while the plastic instruments at about a pound seem to be hard to hold still.

As others have said, I am happy with a mile on land, 3 to 5 at sea. Out of preference I use Pub 249 and the Nautical Almanac. I do keep a long term almanac and 211 in the sextant box as back up. Now days my sextant use is more for fun than real, but.................