The purpose of the multiple shades is rather pedestrian, at least in my view. They are applied, singularly and in combination, until eye relief is obtained. You can approach it the other way, of course. Apply too much shade, removing shades until the observation can be made.
You also commented that you have a replica sextant. Please consider this merely an ornamental device. Without calibration, we cannot identify these errors (1) scale marking (2) centering and (3a) If a drum, the regularity of teeth on arc and drom OR (3b) if a vernier, vernier markings.
NavList has discussed the topic of sextant calibration quite extensively. All of the professional calibration facilities are long closed. Gross sextant errors may be discovered, but I urge you to reconsider an arduous and error prone self calibration, unless you have access to extensive metrology equipment.
The Davis Mark 15 will provide very good results, under the assumption that you verify index error both before and after your round of observations, as these devices are highly affected by temperature changes.
The ordinary slip-stick slide rule may perform some of the spherical trig. Whst you may wish to consider is a cylindrical slide rule to get sufficient digits. The Fuller will certainly turn the trick. Moreover, the Bygrave is a special purpose cylindrical rule, manufactured explicitly for celestial navigation. The "Flat Bygrave" is an excellent, cost effective substitute for a pricey antique
I'm a n00b user/amateur astronav that dived into astro navigation after stumbling over a Bris sextant and wondering if one could ever perform LOP calculatons using a slide rule (got a few of those) :-) (*) So I soon acquired a small shiny King Hughes 1917 replica and recently a Davis Mk 15, and I can happily live with their shortcomings (found an article on how to deal with those shortcomings, and I still have to calibrate these three).
My question is: what is the logic behind the different shades? I understand that one needs more shades for the index mirror (4 vs 3 for the horizon), but why were those colours (blue, blue, orange, grey) chosen? I suspect that the light grey colour can be used for moderate lighting or to increase contrast, and it also reminds of lunar light conditions; but the two blue glasses look similar (I guess a main function is to block out UV), but are there intended ways to combine those glasses?
(*) for now, my answer is that a SR can serve for interpolations; it doesn't have enough digits to provide sufficient accuracy. or maybe with A LOT of sightings, who knows...
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