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I recall a good 20/20 human eye resolves 1 minute. Correct me if I am wrong, that means a 4 power I believe would give you a quarter of that....  .25'  and a 7 power 1/7 of that....  .14'  which is why I embraced a 7 power for lunars very quickly. .......... the other thing is the sextant itself claims a built in seconds of arc resolution limit ....the best claim 9" by manufacture (High end C & P and Tamaya but then tell you you can't read better than 12" in the fine print), while an astra claims I recall 18", because an aluminum arc can't be ground as precisely as brass. That is all a bit esoteric...as to what it really impacts.....but it implies some backlash error built in.  

Because I started on plastic I got aware of backlash...So when we do the index correction on metal, do I always turn one way, or do I forget the backlash and twiddle both ways.  This has to add a little. Add to this the weather, humidity, filter choice, etc. adn its easy to see how index gets fuzzy sometimes. I find the astra has  one nice feature...color bias filters so the reflected and direct sun can be different shades.  Something that frustrates me with the C & P, which lacks this feature, just because I like it.

I found the scope method, going only CCW, produced the ability to get zeroed out to practically no discernable error at 20 x.  Back to 7 power, I seemed on distant objects to stay reliably within +/- .1 on test, going one way only on a hard dock with no movement. I kept landing right near zero..sometimes on, sometimes off +/-.   This may be as good as my horrid technique permits. It was damp warm winter air..and thus a slight distant haze. I will try the sun method next chance I get...my guess is I will still miss a little because my eyes aren't super precise and all the other accumulated factors.

I find the more I do readings, the closer I get to uniform results. Seems I develop unconscious actions which are more precise through repitition.  Beign a perfectionist by nature, I have a tendancy to overshoot to convince myself I was actually right and then back up...which is counter to best accuracy I am convinced.

For what its worth....read with a skeptical eye and correct me if I missed somewhere......I'd rather be embaressed by my own stupididty than forever wrong.