NavList:

David Pike, you wrote:
"So, 1 unit up and 3600 units along equates to 1 minute correction."

Hmm... Of course, you should really be using 3438. :)
It is exceptionally accurate to use
   m.o.a./3438 = size/distance,
where m.o.a. is the angle in minutes of arc. For angles up to about 2°, that is accurate for almost any problem to about one part in 10,000. For angle up to nearly 5°, that's accurate to one part in 1000, compared to a seemingly more "rigorous" trigonometric computation.

Your 3600 rule is only wrong by 5% (and always wrong by 5%), but why even accept that error when you can easily get it right to 0.01%... It makes no difference for the very tiny angle you describe in your message (1.8').

As far as the methodology you used:
"Therefore, if the vertical distance between mirrors is 5cm and the horizontal distance to the roofs is 100m the correction is 5x3,600/10,000 = 1.8’. So, if your real index error is zero, your sextant would be reading minus 1.8,’ and you would have to add that to get a true index error reading. I.E. If you measured minus 2.8’ your actual index error is minus 1’, which is OK for fun astro with an EBBCO, but probably not good enough for serious work."

Sounds reasonable! That method, or a small variant of it, can certainly be made to work. Removing the instrument parallax is a solvable problem. But can you do it when the range of workspace available to you is, let's say, 30 feet? And is there a way to do it that requires little or no calculation at all [and no questions of 3600 versus 3438 :)]. Something that could be accomplished by any navigator under almost any circumstances with any marine sextant from a basic plastic sextant by Ebbco or Davis to a high-end C.Plath... I suspect that the trickier part is discovering the correct lightpath to lightpath distance to use for the instrument parallax. Is it 5cm or 4.5??

Frank Reed