I am glad you seem to have found your solution.
Regarding accuracy with a plastic sextant... there are a few things that will help. First, realize that actual potential accuracy is quite usable, if you do your part.
First of all, remember, you do not use a height of eye, or dip, correction. Second, refraction, parallax, and other optical errors are based on HALF the Ha, which is the angle reading of the sextant corrected for index error, and dip, which is not used with an artificial horizon. Don't just take the Hs and apply the altitude corrections based on the whole angle. This is a big difference from taking a normal sight from the sea horizon. Applying the corrections wrong can introduce several miles of error.
Second, always when you adjust the micrometer knob position, let the final movement be always in the same direction. This way, the real world gear
lash will be somewhat consistent and will just be lost
within the index error. Even better, make your final tweak of the knob set the image and reflected image slightly apart, so that within a half minute or so, the motion of the sun brings them together, and instantly note the time. A hack watch is useful for this, but you can simply count three or five or whatever seconds as you bring your watch into viewing position and note the time, then subtract those few seconds before correcting time to actual GMT. Best practice is to note the watch error and apply it, rather than to be constantly setting the watch exactly to GMT. For purposes of backyard celestial, it is okay to use a radio
controlled "atomic" watch but you do need to still do a radio
Try to keep your sextant in the shade, if possible even when you make your observation. Give it time to stabilize in outdoor temperatures and check your index error immediately before and after the sight. Temperature changes can affect a sextant's reading to a sometimes measurable amount, but particularly so with plastic sextants. The Davis sextants are surprisingly well made and accurate, but temperature sensitivity is one of their major weaknesses. If you make an effort to minimize the effects of temperature, you will be rewarded with more accurate sights.
IMHO it is important to be in the habit of applying an index correction, so setting the index on the sextant itself should be done only ONCE, and you should make the resulting error always be in the same direction so you don't inverse it by mistake. So set it up so that you read a full minute of angle greater than the actual angle of zero when observing this. So if your Hs when taking a sight is say a half minute off, there will be no ambiguity. It will be obvious whether to add or subtract index error.
If you do your part, you should be able to compute and plot an LOP that is within a mile of your actual position, and maybe better than that, with practice. That's not too shabby, huh? A quarter mile accuracy is perhaps attainable.
At sea, a lot of your errors will be lost
in the noise
of the irregular horizon, the bouncy boat
and resulting error in dip correction, and other things. So backyard practice in a controlled setting will help you to avoid making errors you might not notice at sea, giving you a slight edge in accuracy. FWIW, on a small boat
I always endeavor to take the actual sight from the top of a wave, since the horizon is made up of the tops of waves.
In the initial stages it is okay to be a one trick pony, working just on say the mid morning and mid afternoon sun lines. Sun lines are pretty basic, and you have a very wide time window for observing. Other types of sights can wait until you have achieved a good familiarity with the sextant and the math involved. Unfortunately LAN can be problematic, with the artificial horizon, due to the very large doubled angle, but it is nice and simple with the natural horizon. So since you are practicing so much in the backyard, I suggest concentrating on those Sun LOPs first, at home and on the boat
, THEN learning
to do LOP on the boat, and only then graduate to stars, planets, etc, maybe leaving moon for last, even though it is daytime visible and can give you a nice crossing with a Sun line. Moon is slightly more complex and difficult to get right.